Swarm Pokemon
Farfetch'd- Route 1
Volbeat(B2)/Illumise(W2)- Route 3
Natu- Route 5
Plusle(B2)/Minun(W2)- Route 6
Furret- Route 7
Quagsire- Route 8
Swalot- Route 9
Masquerain- Route 11
Doduo- Route 12
Swallow- Route 13
Fearow- Route 15
Pineco- Route 16
Hopip- Route 18
Sudowoodo(B2)/Mr. Mime(W2)- Route 20
Ariados(B2)/Ledian(W2)- Route 22
Hypno- Dreamyard
Hippowdown- Desert Resort
Slowpoke- Abundant Shrine
Cacturne- Reversal Mountain
Legendaries
Zekrom(B2)/Reshiram(W2)- Dragonspiral Tower
Kyurem- Giant Chasm
Cresselia- Marvelous Bridge (Lunar Wing obtained from Strange House)
Latios(B2)/Latias(W2)- Dreamyard
Uxie- Nacrene Museum (After visiting Cave of Being)
Mesprit- Celestial Tower (After visiting Cave of Being)
Azelf- Route 22 (After visiting Cave of Being)
Regirock- Underground Ruins
Registeel(B2)/Regice(W2)- Underground Ruins (Requires Iron Key/Iceberg Key)
Regigigas- Twist Mountain (Requires all three Regi's)
Heatran- Reversal Mountain (Requires Magma Stone)
Hidden Grottos
Nidoran♂♀/Granbull- Route 2
Manectric/Pachirisu/Venonant/Lombre/Bibarel- Route 3
Kingler/Dragonite/Chatot- Route 18
Poliwhirl/Hariyama/Medicham/Bagon- Pinwheel Forest
Beedrill(B2)/Butterfree(W2)/Murkrow/Breloom- Inner Pinwheel Forest
Super Rod Encounters
Feebas/Milotic- Route 1
Golden/Seaking- Routes 3, 11, 14, 22, Abundant Shrine, Striation City, Pinwheel Forest, Aspertia City
Krabby/Kingler- Virbank City/Complex
Clamperl/Huntail(B2)/Gorebyss(W2)- Route 4
Finneon/Lumineon- Route 4, 17, 18, P2 Laboratory, Virbank City, Virbank Complex
Qwilfish- Route 4, P2 Laboratory, Virbank City, Virbank Complex
Relicanth- Route 5
Poliwag/Poliwhrl- Route 6, 19, 20, 23, Giant Chasm, Lostlorn Forest, Wellspring Cave, Victory Road, Floccesy Ranch, Clay Tunnel, Relic Passage
Politoed- Route 6, 19, 20, Lostlorn Forest, Victory Road, Floccesy Ranch
Poliwrath- Route 23, Giant Chasm, Wellspring Cave, Victory Road, Clay Tunnel, Relic Passage
Barboach/Whiscash- Route 8, Icirrus City, Moor of Icirrus
Shellder/Cloyster- Route 13, Undella Town, Humilau City, Seaside Cave
Luvdisc- Route 13, 21, Undella Town, Humilau City, Seaside Cave
Horsea/Seadra/Kingdra- Route 17, 18, P2 Laboratory
Remoraid/Octillery- Route 21, Undella Bay
Chinchou/Lanturn- Undella Bay
Carvanha/Sharpedo- Village Bridge
Dratini/Dragonair/Dragonite- Dragonspiral Tower
In-game trades
Rotom- Route 15 (Trade Ditto)
Ambipom- Accumula Town (Trade Excadrill)
Alakazam- Accumula Town (After you trade Excadrill with the trainer, she'll offer Alakazan in return for your Hippowdon)
Meowth(Yancy)/Mankey(Curtis)- Nimbasa City (After completing the Yancy/Curtis side-quest; accepts any pokémon)
Wobbuffet- Nimbasa City (After completing the Yancy/Curtis side-quest; accepts any pokémon)
Ralts- Nimbasa City (After completing the Yancy/Curtis side-quest; accepts any pokémon)
Shieldon(Yancy)/Cranidos(Curtis)- Nimbasa City (After completing the Yancy/Curtis side-quest; accepts any pokémon)
Rhyhorn- Nimbasa City (After completing the Yancy/Curtis side-quest; accepts any pokémon)
Shellos- Nimbasa City (After completing the Yancy/Curtis side-quest; accepts any pokémon)
Mawile(Yancy)/Sableye(Curtis)- Nimbasa City (After completing the Yancy/Curtis side-quest; accepts any pokémon)
Spiritomb- Nimbasa City (After completing the Yancy/Curtis side-quest; accepts any pokémon)
Snorlax- Nimbasa City (After completing the Yancy/Curtis side-quest; accepts any pokémon)
Teddiursa(Yancy)/Phanpy(Curtis)- Nimbasa City (After completing the Yancy/Curtis side-quest; accepts any pokémon)
Spinda- Nimbasa City (After completing the Yancy/Curtis side-quest; accepts any pokémon)
Togepi- Nimbasa City (After completing the Yancy/Curtis side-quest; accepts any pokémon)
Gifts
Shiny Gible(B2)/Shiny Dratini(W2)- Floccesy Town (After completing Black Tower/White Treehollow)
Happiny- Nacrene City Gate (An egg from a Breeder)
Magikarp- Marvelous Bridge (NPC sells you a Magikarp for 500¥)
Fossils
Omanyte(Helix Fossil)- Twist Mountain (NPC gives you a random fossil daily, revive it in Nacrene City)
Kabuto(Dome Fossil)- Twist Mountain (NPC gives you a random fossil daily, revive it in Nacrene City)
Aerodactyl(Old Amber)- Twist Mountain (NPC gives you a random fossil daily, revive it in Nacrene City)
Lileep(Root Fossil)- Twist Mountain (NPC gives you a random fossil daily, revive it in Nacrene City)
Anorith(Claw Fossil)- Twist Mountain (NPC gives you a random fossil daily, revive it in Nacrene City)
Cranidos(Skull Fossil)- Twist Mountain (NPC gives you a random fossil daily, revive it in Nacrene City)
Shieldon(Armor Fossil)- Twist Mountain (NPC gives you a random fossil daily, revive it in Nacrene City)
Archen(Plume Fossil)/Tirtouga(Cover Fossil)- Nacrene City (Lenora gives one fossil, you decide)
Others
Jigglypuff/Wigglytuff- Route 1, 2, Dreamyard
Lickitung/Lickilicky- Route 2
Yanma/Yanmega- Route 3, Pinwheel Forest
Vigoroth/Slaking- Pinwheel Forest
Croagunk- Route 8, Icirrus City, Moor of Icirrus
Toxicroak- Pinwheel Forest
Lairon- Clay Tunnel, Underground Ruins
Palpitoad/Seismitoad- Route 8, Icirrus City, Moor of Icirrus, Pinwheel Forest
Pupitar/Tyranitar- Route 15
Sources: 1, 2, 3, 4, 5, and Unova location guides on this website.
NOTE: I included Pokémon that are only available post-game, not including evolutions of Pokémon that already have pre-evolutions before post-game and Pokémon that are already available before post-game.
