You cannot get it in Pokemon Silver, but you can in SoulSilver or HeartGold. You have to go to Route 36.
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you have to wait for Alan to call you on your pokegear about finding something. Then you find him and talk to him and he'll give a fire stone. It's takes quite some time for him to call about that.
I geted it from Alan Schoolboy.
If you mean Fire Stone, you can get one by talking to Schoolboy Alan on the route between Goldenrod and Ecruteak, before the spot where Sudowoodo is. Talk to him, get his number, and eventually, he'll call you saying he has a fire stone.
Talk to Alan the Schoolboy on route 36. Battle him and get his number. He is the one with the fire stones.
Keep changing the day-light savings time until Alan calls you about a fire stone. *Note you must rematch him in a battle at least once before he calls about the fire stone. After that you can just ignore the re-battles.