I know there is like a specific shop, but I don't remember where it is D:
If you mean the evolution stones, then go to Lumoise City, there, get a Lumi Cab and find the option Stone Emporium. The thing is, they only sell fire, water and grass stones and the man there sells you a mega stone. If you want (for example) a dusk stone, you can find it with a dowsing machine or at the glittering cave or in super training.