How do you find a fire stone in Pokemon diamond?

You can locate Fire Stones in the Fuego Ironworks. To find Fuego Ironworks, go to the route east of Floaroma Town and surf on the river. Follow the river to Fuego Ironworks. Mr. Fuego should give you a Fire Stone and you can find one on the ground inside.

You could always go underground, too, if you have the Explorer Kit. Good luck!