Where is prize exchange on Pokemon LeafGreen version?

There is no "Prize Exchange" in Pokémon LeafGreen version. You might be thinking of the "Prize Corner" which is located in Celadon City in the Game Corner.

Here's how to find it:

1. Enter the Game Corner: The Game Corner is a large building located in the west part of Celadon City.

2. Go through the entrance: The Prize Corner is located in the back of the Game Corner, behind all the gambling machines.

Here, you can exchange Game Corner Coins that you earn playing the slot machines for various prizes, including rare Pokémon and items.