The Pokemon are sorted earliest availability, but includes all possible places to encounter within the list. Only includes wild encounters and gift encounters; no evolutions (unless they're found in the wild).
Bulbasaur - Starter
Weedle - Routes 2, 24, and 25, Viridian Forest, Pattern Bush
Kakuna - Routes 24 and 25, Viridian Forest, Pattern Bush
NidoranF - Route 3, Safari Zone
NidoranM - Route 3, Safari Zone, Trade Nidoran♀ in Underground Path (Kanto Routes 5–6)
Zubat - Mt. Moon, Rock Tunnel, Seafoam Islands, Victory Road, Icefall Cave, Lost Cave, Altering Cave
Bellsprout - Routes 5, 6, 7, 12, 13, 14, 15, 24, and 25, Berry Forest, Bond Bridge, Cape Brink, Water Path
Gastly - Pokémon Tower, Lost Cave
Haunter - Pokémon Tower, Lost Cave
Venonat - Routes 12, 13, 14, and 15, Berry Forest, Bond Bridge, Safari Zone
Weepingbell - Routes 12, 13, 14, and 15, Berry Forest, Bond Bridge, Cape Brink, Water Path
Tentacool - Routes 4, 10, 11, 12, 13, 19, 20, 21, and 24, Treasure Beach, Kindle Road, Bond Bridge, Five Isle Meadow, Memorial Pillar, Water Labyrinth, Resort Gorgeous, Water Path, Green Path, Outcast Island, Tanoby Ruins, Pallet Town, Cerulean City, Vermilion City, One Island, Five Island, Icefall Cave, Trainer Tower, Cinnabar Island (Surfing)
Nidorina - Safari Zone
Nidorino - Safari Zone, trade Nidorina on Route 11
Venomoth - Safari Zone, Berry Forest
Golbat - Cerulean Cave, Icefall Cave, Lost Cave, Seafoam Islands, Victory Road
Grimer - Pokémon Mansion, Celadon City (Super Rod)
Muk - Pokémon Mansion
Koffing - Pokémon Mansion, Celadon City (surfing)
Tentacruel - Treasure Beach, Kindle Road, Bond Bridge, Five Isle Meadow, Memorial Pillar, Water Labyrinth, Resort Gorgeous, Water Path, Green Path, Outcast Island, Tanoby Ruins, One Island, Five Island, Icefall Cave, Trainer Tower (Surfing)
Spinarak - Pattern Bush
Source: went through every page of this guide, and then each respective Pokémon's page. I highly recommend playing alongside that guide as it has all battles, items, and Pokemon you can find listed by area.
