The Fire Stone is available in 2 places in Pokemon Crystal:
First, from Schoolboy Alan (Route 36, go south from Sudowoodo spot) after beating him in a rematch. Register his phone number in order to have a rematch. He should be the first person south of the Sudowoodo.
Secondly, from Bill's Grandfather (Route 25, Cottage on Cape). You will have to show him several Pokemon, at some point he'll ask to see a Growlithe or Vulpix. Show that Pokemon to him and he will give you a Fire Stone. Note that this can only be done After beating the Final Four in Johto, and that this is the ONLY way to get a Fire Stone in Gold/Silver.
Chat with our AI personalities
Talk to Alan the Schoolboy on route 36. Battle him and get his number. He is the one with the fire stones.
In Pokemon Crystal.
What the hell is a Pokemon crystal
There is no Groudon in Pokémon Crystal.
You can find a Fire Stone in the Fiery Path after you get the HM for Strength.