From Bulbapedia, the community-driven Pokémon encyclopedia.
This is a list of Pokémon obtainable from events found in the Generation I Pokémon games. These specific instances of Pokémon can only be obtained once per game; however, some may also be obtained in other ways as well.
A Poké Ball containing Eevee can be found on the roof floor of Celadon Mansion in Celadon City.
There are two set encounters with Snorlax accessible via the Poké Flute; one on Route 12 and one on Route 16. The player must capture or defeat at least one of them to proceed to Fuchsia City.
After defeating Blue during Team Rocket's invasion of Silph Co., an onlooking Silph employee will entrust the player with a Lapras.
Articuno is found within the depths of the Seafoam Islands.
Zapdos is found inside the abandoned Power Plant.
Moltres can be found in Victory Road.
Mewtwo is found within the depths of Cerulean Cave, accessible only to powerful Trainers who have defeated the Elite Four.
Bulbasaur will be given to the player by a girl in Cerulean City if the player's starter Pikachu has high enough happiness.
A boy north of Route 24 will give the player a Charmander.
An Officer Jenny in Vermilion City will give the player a Squirtle after defeating Lt. Surge