Can you get Eevee on Pokemon FireRed or Emerald and how?

You can't get Eevee in Pokemon FireRed or Emerald.

Eevee was introduced in Generation I (Pokemon Red/Blue/Yellow), but wasn't available in Generation III (Ruby/Sapphire/Emerald or FireRed/LeafGreen).

Therefore, you can't find or catch Eevee in those games.