You can't buy any of the elemental stones sorry
Here are the locations of the stones;
Water Stone; Dustclouds in caves, Castelia City (You must answer Panpour), Driftveil City near the lighthouse
Fire Stone: Dustclouds in caves, Castelia City (You must answer Pansear), Desert Resort
Thunder Stone: Route 9 (Department Store), Chargestone Cave, Dustclouds in caves
Alternatively you can trade. That is all sorry.