Not including transfer or Dream World.
Butterfree (White) - Shaking Grass in Route 12
Beedrill (Black) - Shaking Grass in Route 12
Pidgey (White) - White Forest
Raticate - Dreamyard
Fearow - Route 15
Sandslash - Relic Castle
Nidoran (White) - White Forest
Clefairy - Giant Chasm
Clefable - Shaking Grass in Giant Chasm
Vulpix - Abundant Shrine
Ninetails - Shaking Grass in Abundant Shrine
Jigglypuff - Route 14
Wigglytuff - Shaking Grass in Route 14
Golbat - Route 13, Giant Chasm
Oddish (White) - White Forest
Paras (White) - Swarm in Route 11
Golduck - Route 11, Route 14, Village Bridge
Mankey - Swarm in Route 15
Poliwag - Use Super Rod in Route 6, Wellspring Cave, Challenger's Cave, Victory Road, Giant Chasm
Poliwhirl - Use Super Rod in Route 6, Wellspring Cave, Challenger's Cave, Victory Road, Giant Chasm
Poliwrath - Use Super Rod in Wellspring Cave, Challenger's Cave, Victory Road, Giant Chasm
Abra - White Forest
Machop - White Forest
Bellsprout - White Forest
Graveler - Challenger's Cave
Rapidash - Route 12
Slowpoke - Surf in Abundant Shrine
Slowbro - Surf in Abundant Shrine
Magnemite - White Forest
Farfetch'd - Swarms in Route 1
Doduo - Swarms in Route 12
Seel - Surf in Giant Chasm
Dewgong - Surf in Giant Chasm
Shellder - Use Super Rod in Route 13, Undella Town, Undella Bay
Cloyster - Use Super Rod in Route 13, Undella Town, Undella Bay
Gastly - White Forest
Onix - Relic Castle
Krabby - Use Super Rod in Route 4, Route 13, Driftveil City
Kingler - Use Super Rod in Route 4, Route 13, Driftveil City
Exeggcute - Swarms in Route 18
Marowak - Route 15
Lickitung - Challenger's Cave
Rhyhorn - White Forest
Tangela - Route 13, Giant Chasm
Kangaskhan - Route 15
Horsea - Use Super Rod in Route 17, Route 18, and P2 Laboratory
Seadra - Use Super Rod in Route 17, Route 18, and P2 Laboratory
Goldeen - Use Super Rod in Route 3, Route 11, Route 14, Striaton City, Inner Pinwheel Forest, Lostlorn Forest, Abundant Shrine
Seaking - Use Super Rod in Route 3, Route 11, Route 14, Striaton City, Inner Pinwheel Forest, Lostlorn Forest, Abundant Shrine
Staryu- Surf in Route 13
Starmie- Surf in Route 13
Jynx - Giant Chasm
Pinsir - Route 12
Magikarp - Buy from Magikarp Salesman on Marvelous Bridge for 500 Pokédollars
Lapras - Surf in rippling water in Village Bridge
Ditto - In Plains of Giant Chasm
Porygon - White Forest
Omanyte - Revive from Helix Fossil (receive from Worker in Twist Mountain)
Kabuto - Revive from Dome Fossil (receive from Worker in Twist Mountain)
Aerodactyl - Revive from Old Amber (receive from Worker in Twist Mountain)
Dratini- Use Super Rod outside of Dragonspiral Tower
Dragonair - Use Super Rod in bubbling water outside of Dragonspiral Tower
Dragonite - Use Super Rod in bubbling water outside of Dragonspiral Tower
Sentret - Swarms in Route 7
Noctowl - Abundant Shrine
Ledian- Dreamyard
Ariados - Dreamyard
Crobat- Route 13, Shaking Grass outside of Giant Chasm
Chinchou - Use Super Rod in Route 18, Driftveil City
Lanturn - Use Super Rod in bubbling water in Driftveil City
Togepi - White Forest
Mareep - White Forest
Politoed - In Bubbling Water in Route 6
Hoppip - White Forest
Sunkern - Route 12
Sunflora - In Shaking Grass in Route 12
Yanma- Swarms in Route 14
Wooper - Surf in White Forest
Murkrow - (Black only) Abundant Shrine
Slowking - Surf in Abundant Shrine
Misdreavus (White only) - Abundant Shrine
Pineco - Swarms in Route 16
Dunsparce - Route 12
Gligar - Route 11, Route 15
Qwilfish - Bubbling Water in Route 17, Route 18, and P2 Laboratory
Shuckle - Route 14
Heracross - Route 12
Sneasel - Giant Chasm
Piloswine - Giant Chasm
Corsola - Surf in Route 13, Undella Town
Remoraid - Fish in Undella Town, Undella Bay
Octillery - Fish in Undella Town, Undella Bay
Delibird - Giant Chasm
Mantine - Surf in Undella Town, Undella Bay
Houndour - (Black only) Swarms in Route 9
Kingdra - Bubbling water in Route 17, Route 18, and P2 Laboratory
Stantler - Abundant Shrine
Smeargle - Swarms in Route 5
Tyrogue - Swarms in Route 10
Elekid - White Forest
Magby - White Forest
Pupitar - Route 15
Tyranitar - Route 15
Poochyena - (White only) Route 9
Wurmple - White Forest
Lotad - Surf in White Forest
Seedot - White Forest
Swellow - Route 13, Giant Chasm
Wingull - Surf in Route 13, Undella Town, Undella Bay
Pelipper - Surf in Route 13, Undella Town, Undella Bay
Ralts - White Forest
Surskit - Surf in White Forest
Shroomish - (Black only) Swarms in Route 11
Slakoth - White Forest
Whismur - White Forest
Azurill - White Forest
Sableye - Challenger's Cave
Mawile - Challenger's Cave
Aron - White Forest
Plusle - (Black only) Swarms in Route 6
Minun - (White only) Swarms in Route 6
Volbeat - (In Black) Swarms in Route 3
Illumise - (White only) Swarms in Route 3
Carvanha - Use Super Rod on Village Bridge
Sharpedo - Use Super Rod in bubbling water on Village Bridge
Wailmer - Surf in Undella Town, Undella Bay
Wailord - Surf in Undella Bay
Trapinch - White Forest
Altaria - Route 14
Zangoose - Route 11, Village Bridge
Seviper - Route 11, Village Bridge
Lunatone - Route 13, Giant Chasm
Solrock - Route 13, Giant Chasm
Barboach - Use Super Rod in Route 8, Icirrus City, Moor of Icirrus
Whiscash - Use Super Rod in bubbling water in Route 8, Icirrus City, Moor of Icirrus
Corphish - Surf in White Forest
Claydol - Sanctum of Relic Castle
Lileep- Revive from Root Fossil (receive from Worker in Twist Mountain)
Anorith - Revive from Claw Fossil (receive from Worker in Twist Mountain)
Feebas - Use Super Rod in Route 1 and in bubbling water in Route 1
Milotic- Use Super Rod in bubbling water in Route 1
Shuppet - Swarm in Route 13
Tropius - Route 14
Chimecho -Abundant Shrine
Absol- Route 13, Giant Chasm
Wynaut - Swarm in Route 2
Spheal- Surf in Undella Bay in Winter only
Sealeo - Surf in Undella Bay in Winter only
Walrein - Surf in Undella Bay in Winter only
Clamperl - Use Super Rod in Route 4
Huntail - (In Black) Use Super Rod in Route 4,
Gorebyss - (In White) Use Super Rod in Route 4
Relicanth - Use Super Rod in bubbling water in Route 4
Luvdisc - Use Super Rod in regular or bubbling water in Route 4, Route 13, Driftveil City, Undella Town, Undella Bay
Bagon - White Forest
Metang - Plains of Giant Chasm
Metagross - Shaking grass in Giant Chasm
Bibarel Village - Bridge
Shinx - White Forest
Budew - White Forest
Cranidos - Revive from Skull Fossil (receive from Worker in Twist Mountain)
Shieldon - Revive from Armor Fossil (receive from Worker in Twist Mountain)
Combee - Route 12
Vespiquen - Shaking grass in Route 12
Buizel - Surf in regular and bubbling water in Route 11, Route 14
Floatzel - Surf in Route 11, Route 14
Cherrim - Route 12
Drifblim - Route 13, Route 14, Giant Chasm
Mismagius - (White only) Shaking grass in Abundant Shrine
Honchkrow - (Black only) Shaking grass in Abundant Shrine
Chingling - Breed Chimecho
Bronzong - Abundant Shrine
Happiny - White Forest
Munchlax - Trade Cincinno in Undella Town in Summer only
Riolu - Challenger's Cave
Hippopotas - Swarm in Route 4
Croagunk - Swarm in Route 8
Finneon - Use Super Rod in Route 17, Route 18, P2 Laboratory
Lumineon - Use Super Rod in bubbling water in Route 17, Route 18, P2 Laboratory
Mantyke - Surf in Undella Town, Undella Bay
Tangrowth - Route 13, Giant Chasm
Gliscor - Shaking grass in Route 11, Route 15
Mamoswine - Shaking grass in Giant Chasm
Rotom - Trade Ditto in Route 15
Source