Where do you buy an evee in firered?

You can't buy an Eevee in FireRed. Eevee is not available in the game. You can, however, obtain an Eevee through trading with someone who owns a copy of LeafGreen.

Here's why:

* Pokémon in FireRed and LeafGreen: Each game in the Generation III games (FireRed, LeafGreen, Ruby, Sapphire, Emerald) features different Pokémon.

* Eevee's Exclusivity: Eevee is exclusive to LeafGreen.

If you really want an Eevee in your FireRed game, you'll need to trade with someone who has LeafGreen.