My friend wants to know where to get a fire stone so he can evolve his growlithe into arcanine
There are many ways to get a fire stone.
Bill's grandfather. He's located at Cerulean City and you show him a Growlithe if you have HG and Vulpix if you have SS
You can get Schoolboy Alan's(Route 36) phone number and he will tell you that he found a item and it would be a Fire Stone
You can win one by getting 1st place in the Bug-Catching contest
You can do the Pokeathlon Dome and like Fritjof said they are available on certain days. (Tues before National Dex and Sun and Thurs after National Dex)
You can find them on the Pokewalker on Warm Beach. You have to take about 5000+ steps.