Z-Fighters are hard to find. if you want a certain fighter, look no further!
Fighters with only one Encounter
There are 8 fighters that can only be encountered and captured once. Four (4) of them are the movie villains that are shown on-screen as opposed to being randomly encountered: Broly (#071), Bojack (#074), Hatchiyack (#078), and Hirudegarn (#102). All of them are Level 45. The other four (4) are Shenron (#116) at Level 60, and Beerus (#117), Whis (#118), and Omega Shenron (#136) at Level 70. If you flee the battle or defeat them, they will disappear forever. There is no special benefit to defeating them, exp-wise or otherwise, so be sure to capture them unless you're entirely sure you don't want to.
It's highly recommended to use a save state just before you approach each one, as you can reload to that state if you fail to capture the fighter, either because of death, victory, or running out of capsules. It takes about 20-40 Capsules to successfully capture one of these fighters at <10% health, so be sure to lower their HP before using up your capsules. Another useful tactic is to lower their HP to the lowest possible, and make a save state there. That way, you can reload to the same spot in the battle to try all your capsules again, increasing your odds of success.
Trades
Note: Trades can be done once and are the same level as the Fighter that you offer. Units you get in a trade gain extra experience
Key: Give → Get
Frieza → Cooler - House in Namek Town
Slug → Kid Gohan - Route 2 (Through R.R. Cave)
Bardock → King Vegeta - Between Ginger Town and Route 18
Majin Buu → Janemba - Roshi's Island Science Lab Room 1
Perfect Cell → Dabura - Roshi's Island Science Lab Room 1
Gotenks → Gogeta - Roshi's Island Science Lab Room 2
Oosaru → Slug - Route 5 (underground cave)
Ginyu → Sauzer - Route 11 (Enter and up the stairs first floor)
Taopaipai → Zarbon 2 House In Red Ribbon Base
Fighter List
No.001: Goku - Starter (Lvl 5), Sevii Islands 5 and 7 post-game
No.002: SSJ Goku - Evolve from Goku at Lvl 16, Sevii Island 7 post-game
No.003: SSJ3 Goku - Evolve from SSJ Goku at Lvl 36
No.004: Vegeta - Starter (Lvl 5), Sevii Islands 5 and 7 post-game
No.005: SSJ Vegeta - Evolve from Vegeta at Lvl 16, Sevii Island 7 post-game
No.006: Majin Vegeta - Evolve from SSJ Vegeta at Lvl 36
No.007: Gohan - Starter (Lvl 5), Sevii Island 6 post-game
No.008: SSJ Gohan - Evolve from Gohan at Lvl 16, Sevii Island 6 post-game
No.009: Ultimate Gohan - Evolve from SSJ Gohan at Lvl 36
No.010: Piccolo - North Forest (Lvl 3-6), Route 4 (Lvl 8-10), Route 5 (Lvl 14-15)
No.011: Super Piccolo - Evolve from Piccolo at Lvl 20, Berry Forest (Lvl 37-38)
No.012: Krillin - Route 22 (Lvl 3-4), Route 21 (Lvl 20-25), Kindle Road (Lvl 30-31)
No.013: Yamcha - Route 22 (Lvl 3-4), Route 7 (Lvl 16-17)
No.014: Tenshinhan - Mt. Moon (Lvl 6-9), Route 12 (Lvl 23-27), Outside Mt. Ember (Lvl 33-38), Mt. Ember Cave (Lvl 32-37)
No.015: Chiaotzu - Route 4 Healing Center for 500 Zeni (Lvl 5), Route 12 (Lvl 22-26)
No.016: Bear Thief - Route 1 (Lvl 2-6), Route 8 (Lvl 17-20)
No.017: Giran - Mt. Moon (Lvl 6-11), Route 8 (Lvl 17-20), Mt. Ember Cave (Lvl 29-36)
No.018: Buyon - Red Ribbon Cave (Lvl 15-16), Rock Tunnel (Lvl 16)
No.019: Eighter - Red Ribbon Cave (Lvl 15-16)
No.020: Taopaipai - Game Corner 200 coins (4000 Zeni) (Lvl 7)
No.021: Drum - North Forest (Lvl 3-5), Berry Forest (Lvl 30/34)
No.022: Tanbarin - North Forest (Lvl 3-5), Berry Forest (Lvl 32/34)
No.023: King Piccolo - God Tower (Lvl 13-25), Enigma Mansion (Lvl 30-32)
No.024: Saibaman - Route 1 (Lvl 2-4), North Forest (Lvl 2-6), Route 22 (Lvl 2-4), Route 3 (Lvl 5), Route 8 (Lvl 16-17), Route 12 (Lvl 22), Babidi Zone (Lvl 20-25), Enigma Mansion (Lvl 26-30), Outside Mt. Ember (Lvl 29-31)
No.025: Raditz - Route 2 (Lvl 2-5), Route 3 (Lvl 6-7), Mt. Moon (Lvl 2-9), Kindle Road (Lvl 30-34) (Moon Stone -> Oosaru)
No.026: Nappa - Route 3 (Lvl 6-8), Mt. Moon (Lvl 8-9) (Moon Stone -> Oosaru)
No.027: Turles - Route 2 (Lvl 2-5) (Moon Stone -> Oosaru)
No.028: Bardock - Game Corner 2,800 coins (56,000 Zeni) (Lvl 15) (Moon Stone -> Oosaru)
No.029: SSJ Bardock - Evolve from Bardock at Lvl 25
No.030: King Vegeta - Via trade (offer Bardock), Route 8 (Lvl 19-20) (Moon Stone -> Oosaru)
No.031: Kid Gohan - Route 10 (Lvl 16) (Moon Stone -> Oosaru)
No.032: Oosaru - Mt. Moon (Lvl 10), Route 23 (Lvl 40-42), OR use Moon Stone on Fighters No. 25-30: Raditz, Nappa, Turles, Bardock, King Vegeta, Kid Gohan (Lvl is the same as original Fighter)
No.033: Golden Oosaru - Evolve from Oosaru at Lvl 45
No.034: Kiwi - Route 25 (Lvl 10-12)
No.035: Dodoria - Route 25 (Lvl 11-13), Route 9 (Lvl 13-14), Route 10 (Lvl 13), Outside Mt. Ember (Lvl 32-36)
No.036: Zarbon - Route 25 (Lvl 11-13), Route 9 (Lvl 14), Route 10 (Lvl 14)
No.037: Zarbon 2 - Evolve from Zarbon at Lvl 25
No.038: Guldo - Route 5 (Lvl 10-12), Route 9 (Lvl 11-13), Route 10 (Lvl 14-15), Rock Tunnel (Lvl 15)
No.039: Recoome - Route 9 (Lvl 16-17), Route 10 (Lvl 15-16)
No.040: Jeice - Babidi Zone (Lvl 25/27)
No.041: Burter - Babidi Zone (Lvl 25/27)
No.042: Ginyu - Game Corner 1000 coins (20,000 Zeni) (Lvl 12)
No.043: Frieza - Rock Tunnel (Lvl 17-18), Seafoam Islands (Lvl 27-28)
No.044: Frieza 2 - Evolve from Frieza at Lvl 16, Seafoam Islands (Lvl 29-31), Victory Road (Lvl 42-46)
No.045: Frieza 3 - Evolve from Frieza 2 at Lvl 24, Victory Road (Lvl 44-46)
No.046: Frieza 4 - Evolve from Frieza 3 at Lvl 30, Imperial Cave
No.047: Sauzer - Rock Tunnel (Lvl 14)
No.048: Cooler - Rock Tunnel (Lvl 17-18), Seafoam Islands (Lvl 28-30)
No.049: Super Cooler - Evolve from Cooler at Lvl 30, Imperial Cave
No.050: Metal Cooler - Evolve from Super Cooler -> Metal Coat
No.051: King Cold - Seafoam Islands (Lvl 29-31)
No.052: Nail - Route 4 (Lvl 8-10), Route 24 (Lvl 9-14), Route 5 (Lvl 12-15)
No.053: Slug - Route 24 (Lvl 8-12)
No.054: Garlic - God Tower (Lvl 25), Raichi's Lab (Lvl 25-28)
No.055: Garlic 2 - Victory Road (Lvl 44-45), Evolve from Garlic at at Lvl 28
No.056: Bioman - Route 6 (Lvl 10-12), Route 11 (Lvl 12-13), Rock Tunnel (Lvl 15), Raichi's Lab (Lvl 23-25)
No.057: Wilo - Get Wilo's Brain from the museum in Saiyan Town and revive it on Roshi's Island (7th Badge town)
No.058: C-13 - Route 11 (Lvl 14-17)
No.059: Super C-13 - Evolve from C-13 at Lvl 30
No.060: C-14 - Route 6 (Lvl 13/16)
No.061: C-15 - Route 6 (Lvl 13/15/16)
No.062: C-16 - Capsule Corp (Lvl 25)
No.063: C-17 - Route 16 (Lvl 17-24), Route 14 (Lvl 23-25)
No.064: C-18 - Route 16 (Lvl 17-24), Route 14 (Lvl 23-25)
No.065: C-19 - Route 11 (Lvl 14-15)
No.066: C-20 - Route 11 (Lvl 15-16)
No.067: Bug Cell - Route 7 (Lvl 18-19), Bicycle Road (Lvl 23-25), Kindle Road (Lvl 32/37)
No.068: Imperfect Cell - Evolve from Bug Cell at Lvl 24, Bicycle Road (Lvl 27-28), Kindle Road (Lvl 34/38)
No.069: Perfect Cell - Evolve from Imperfect Cell at Lvl 33, Imperial Cave
No.070: Cell Jr - Routes 13/14/18/17 (Lvl 24-28), Babidi Zone (Lvl 25-26), Raichi's Lab (Lvl 24-26), Berry Forest (Lvl 35-36)
No.071: Broly - Summit of Mt. Ember (Lvl 45) - ONLY CHANCE
No.072: SSJ Broly - Evolve from Broly at Lvl 25
No.073: LSSJ Broly - Evolve from SSJ Broly at Lvl 36
No.074: Bojack - Seafoam Islands (Lvl 45) - ONLY CHANCE
No.075: Bojack 2 - Evolve from Bojack at Lvl 32
No.076: Zangya - Bicycle Road (Lvl 22-24)
No.077: Bujin - Enigma Mansion (Lvl 30-32)
No.078: Hatchiyack - Raichi's Lab (Lvl 45) - ONLY CHANCE
No.079: Future Trunks - Game Corner 5,500 coins (110,000 Zeni) (Lvl 20)
No.080: SSJ Future Trunks - Evolve from Future Trunks at Lvl 16
No.081: Teen Gohan - Route 7 (Lvl 17-18), Cape Brink (Lvl 30-35)
No.082: SSJ Teen Gohan - Evolve from Teen Gohan at Lvl 16, Cape Brink (Lvl 31-38)
No.083: SSJ2 Teen Gohan - Evolve from SSJ Teen Gohan at Lvl 34
No.084: Spopovich - Babidi Zone (Lvl 25)
No.085: Yamu - Babidi Zone (Lvl 25)
No.086: Pui Pui - Babidi Zone (Lvl 22-26)
No.087: Yakon - Babidi Zone (Lvl 24-31)
No.088: Dabura - Babidi Zone (Lvl 26-30), Victory Road (Lvl 40-46)
No.089: Majin Buu - Babidi Zone (Lvl 30-32), Victory Road (Lvl 34-48)
No.090: Evil Buu - Evolve from Majin Buu at Lvl 25
No.091: Super Buu - Evolve from Evil Buu at Lvl 38, Imperial Cave
No.092: Kid Buu - Evolve from Super Buu at Lvl 45
No.093: Kaioshin - Imperial Cave
No.094: Kibito - Imperial Cave
No.095: Kibitoshin - Imperial Cave
No.096: Aqua - Any fishing or swimming spot (Lvl 6-40), Seafoam Islands (Lvl 25-39), Imperial Cave
No.097: Torbie - Imperial Cave, Lvl 50~ to 60~
No.098: Olibu - Imperial Cave, Lv50~ to 60~
No.099: Pikkon - Imperial Cave, Lv50~ to 60~
No.100: Janemba - Via trade for Majin Buu, Victory Road (Lvl 34-48)
No.101: Super Janemba - Evolve from Janemba at Lvl 45, Imperial Cave
No.102: Hirudegarn - Catch it when you awaken him (Lvl 45) - ONLY CHANCE
No:103: Hirudegarn 2 - Evolve from Hildegarn at Lvl 50
No.104: Tapion - Route 23 (Lvl 30-36), Victory Road (Lvl 38/42)
No.105: Goten - Route 15 (Lvl 23-25), Bond Bridge (Lvl 29-34)
No.106: SSJ Goten - Evolve from Goten at Lvl 16, Bond Bridge (Lvl 37)
No.107: Trunks - Route 15 (Lvl 23-28), Bond Bridge (Lvl 30-34)
No.108: SSJ Trunks - Evolve from Trunks at Lvl 16, Bond Bridge (Lvl 38)
No.109: Gotenks - Sold for 9,999 coins in Game Corner (199,980 Zeni) (Lvl 25)
No.110: Gotenks 2 - Evolve from Gotenks at Lvl 23
No.111: Gotenks 3 - Evolve from Gotenks 2 at Lvl 38
No.112: Gogeta - Trade (offer Gotenks, Cinnabar Island), Memorial Pillar
No.113: SSJ Gogeta - Evolve from Gogeta at Lvl 30
No.114: Vegeto - Canyon entrance (south of Seven Island)
No.115: SSJ Vegeto - Evolve from Vegeto at Lvl 30
No.116: Shenron - Collect all 7 Dragon Balls - then talk to old man next to God's Tower (Lvl 60) - ONLY CHANCE
No.117: Beerus - Imperial Cave Basement (Mewtwo's spot) (Lvl 70) - ONLY CHANCE
No.118: Whis - Find Aurora ticket next to the Memorial Pillar - then take the boat to Birth Island (Deoxys spot); Birth Island Capsule (Lvl 70) - ONLY CHANCE (you can fight him multiple times as long as he's not defeated, but you need to repeat the puzzle)
No.119: Golden Frieza - Evolve from Frieza 4 at Lvl 50
No.120: Hit - Tanoby Ruins
No.121: Uub - Water Path
No.122: Super Uub - Evolve from Uub at Lvl 35
No.123: Ledgic - Five Isle Meadow
No.124: Sigma - Ruin Valley
No.125: Rilldo - Ruin Valley
No.126: Mega Rilldo - Evolve from Rilldo at Lvl 40
No.127: Metal Rilldo - Evolve from Mega Rilldo + Metal Coat
No.128: Baby - Sevii Island 6 (Altering Cave)
No.129: Baby Vegeta - Evolve from Baby at Lvl 40
No.130: Super C-17 - Memorial Pillar
No.131: SSJ4 Goku - Evolve from SSJ3 Goku + Ape Tail
No.132: SSJ4 Vegeta - Evolve from Majin Vegeta + Ape Tail
No.133: SSJ4 Gogeta - Evolve from Ssj Gogeta + Ape Tail
No.134: Nova Shenron - Sevault Canyon (south of Seven Island)
No.135: Eis Shenron - Icefall Cave
No.136: Omega Shenron - Sevault Canyon (walk behind the house with the dance man) (Lvl 70) - ONLY CHANCE
No.137: SSJ God Gohan - Requires Gohan as starter - Random encounter in grass area in Kanto (after delivering both the Ruby and Sapphire)
No.138: SSGSS Vegeta - Requires Vegeta as starter - Random encounter in grass area in Kanto (after delivering both the Ruby and Sapphire)
No.139: SSJ God Goku - Requires Goku as starter - Random encounter in grass area in Kanto (after delivering both the Ruby and Sapphire)
No.140: SSGSS Goku - Evolve from SSJ God Goku at Lvl 55