Where do I get a fire stone in heartgold

Bill's grandfather. he's in the house on route 25 near the cape and asks to see a few different pokemon, one at a time. the Fire Stone is his reward for showing him a Growlithe or Vulpix. you have to do them in his order, so you might need to show him the earlier ones first (Lickitung, Oddish, Staryu) before he gets to that one. the Pokeathlon dome also trades stones for athlete points, but what's in stock changes with the day of the week, so check back if it isn't there.