Good news, you can find a fire stone in the desert resort, right next to route 4 as long as you've got the dowsing machine, which, based on your description you already have. It's in the bottom left corner of the desert, by the one of the walls.
As for the water stone, you could have already gotten one, on route 19, one of the first routes in the game, next to Aspertia City(The city where you fought Cheren, the first gym leader)
Sources: Serebii Itemdex and Pokearth pages, and the link that Amethyst provided.