Fire Stone: Fiery Path
Repeatable: Route 124 (Red Shard trader)
Water Stone: Abandoned Ship
Repeatable: Route 124 (Blue Shard trader)
Thunder Stone: New Mauville
Repeatable: Route 124 (Yellow Shard trader)
Leaf Stone: Route 119
Repeatable: Route 124 (Green Shard trader)
Moon Stone: Meteor Falls
Repeatable: Held by wild Lunatone (5% chance)
Sun Stone: Mossdeep Space Center
Repeatable: Held by wild Solrock (5% chance)
Source
Hope this helped