Where do you get a fire stone in the unova region?

You can't get a Fire Stone in the Unova region in the main games (Black, White, Black 2, White 2).

The Fire Stone is an evolution item used to evolve certain Pokemon (like Vulpix, Growlithe, and Eevee). It's available in other regions but not Unova.

If you're playing a Pokemon game where you can trade, you could ask someone who has a Fire Stone to trade it to you. Otherwise, you might need to find a different way to evolve the Pokemon you're aiming for.