Fire Stone
You will find the Fire Stone in Fiery Path, an underground tunnel beneath Mt. Chimney. The stone is on the left side of the cave, behind a boulder that you can only move after you teach the Strength ability to a Pokemon in your party.
Water Stone
The Water Stone is in an abandoned ship beneath Route 108. To reach the underwater treasure, your Pokemon need the Surf and Dive abilities.
Thunder Stone
The Thunder Stone is in New Mauville, a power plant beneath Mauville City. To reach the power plant, go to the east side of Route 110 and use Surf. After you navigate the building, you will find the Thunder Stone in the generator room.
Leaf Stone
You will find the Leaf Stone on Route 119. Travel to the Weather Institute and use Surf to go down the river until you find the stone.
Moon Stone
The Moon Stone is in Meteor Falls, a cave west of Fallarbor Town. The cave is a maze, and the stone is in the main chamber. Wild Lunatone may also carry the stone.
Sun Stone
A man in the Research Center in Mossdeep City has a Sun Stone. You can also get the stone from wild Solrock.
Trading for Stones
The Diving Treasure Hunter lives in a hut on Route 124. If you bring him a colored shard, he will give you an evolution stone. The color of the shard determines which stone he will offer. For example, a Blue Shard earns you a Water Stone, while a Red Shard earns you a Fire Stone. You can find shards underwater after you teach the Dive ability to one of your Pokemon.