Kanto, And Hoeen Starter:

After beating Red on Mt. Silver, go to Pallet town and go to the Pokémon lab and pick one of the three original starters. After that go to Saffron city and visit Silph co. after meeting Steven for the first time at the Pokémon club after talking to the girl in the pink house in Saffron city who lost her Clefairy doll, he will be standing at the counter. When you talk to him he will say a few things then he will ask you if you found three stones which would you keep, green=Treeko, red=Torchic, blue=Mudkip.