Leaf Stones are used to evolve Gloom into Vileplume and Weepinbell into Victreebel. However, these evolutions are triggered by trading in Soul Silver.
Here's how you get them:
* Gloom: Trade a Gloom to someone who has a Sun Stone. They can then evolve it into Vileplume and trade it back to you.
* Weepinbell: Trade a Weepinbell to someone who has a Sun Stone. They can then evolve it into Victreebel and trade it back to you.
Note: You can only evolve these Pokemon by trading them while holding the Sun Stone. You can find a Sun Stone in the following locations:
* Route 34: There's a chance you'll find a Sun Stone in the tall grass.
* National Park: You can find a Sun Stone in the tall grass here.
* Goldenrod Department Store: You can purchase a Sun Stone here for 5,000 Poké Dollars.
So, while you don't need a Leaf Stone to evolve these Pokémon, you'll need to find a Sun Stone and utilize the trading system in Soul Silver to get them.