You are able to purchase a fire stone with battle points and a variety of other evolutionary stones in the Gear Station and PWT.
In Join Avenue, if your Antique Shop sells a Dirty Stone (Shop Rank 2), Black Stone (Shop Rank 5), Dingy Stone (Shop Rank 8), or Dusty Stone (Shop Rank 10), it may contain a fire stone when appraised.
Evolutionary stones can also be found in dust clouds that pop up in caves as well.
As for fire stones on the ground, there is one in Lentimas Town which can be found on the hill north of the Pokemon Center and one in the Desert Resort located north of Psychic Gaven, near the stone walls on the western side.
I hope I helped!
Bulbapedia: Desert Resort
Bulbapedia: Join Avenue
Bulbapedia: Evolutionary Stones
Bulbapedia: Lentimas Town