Pokémon FireRed and LeafGreen/Route 14-15

After passing through Routes 12 and 13, you'll end up in Routes 14 and 15.

Professor Oak's Aide
Inside the Guard Post building on Route 15, east of Fuchsia City, you will find one of Oak's Aides upstairs. If you have caught more than 50 kinds of Pokémon, he will reward you with the EXP Share! Unlike the EXP All in Generation I, this is a held item. Giving this item to a Pokémon allows it to receive half the experience from every battle without bringing it out. The remaining half is divided between the Pokémon that participated. This held item will come in handy if you want to change your team composition but need to level up the newcomer.