Where to find Pikachu and Eevee in the wild | Pokémon Let's Go Pikachu and Eevee
In Pokémon Let's Go, both Eevee and Pikachu become your main starting Pokemon and stay with you for the whole of your journey. Unlike other core RPG games, however, these Pokémon cannot evolve.
If you are looking to complete your Pokédex by catching a Raichu, Vaporeon, Jolteon, or Flareon, then you will need to rely on catching a wild version of these Pokémon first to help you on your quest.
Thankfully, you can catch a these Pokemon in certain areas of the game. Granted, they are mostly rare spawns, so be prepared to wait in these areas for awhile.
Viridian Forest as a rare spawn
Route 17 as a rare spawn