The Firestone in Monster Hunter Wilds is a rare material to find as it's in a very exclusive spot in the game. It's also very dangerous to acquire one as you need to stay chilled to mine this particular mineral.

In Monster Hunter Wilds you can find Carbalite Ore at any Mining Outcrop (except Oilwell Basin) or from Piragill once you reach high rank. You need to complete the first mission of Chapter 4 in the main story to reach high rank, if you look for Carbalite Ore before this point you won't find it.