1. Route 12: This is the most common location to find a Water Stone. Look for it in the tall grass.
2. Trade: You can trade for a Water Stone with other players.
3. Pokemon Dream World: If you have access to the Dream World, you can sometimes find Water Stones there.
4. Find it on the Ground: You can occasionally find Water Stones on the ground throughout the game. Just keep your eyes peeled!
Remember that Water Stones are relatively rare, so you might have to search a bit before you find one.