There are a few ways to obtain it:
Johto: Come first in the National Park's competition; Route 36, defeat a specific trainer multiple times and he will give you one
Kanto: Route 25, show Bill's grandfather the requested fire-type pokemon
Pokewalker: Warm Beach (after 5000 steps)
And , of course, trading from another game.
Related links:
http://serebii.net/pokearth/johto/route36.shtml#4th
http://serebii.net/pokearth/kanto/route25.shtml#4th
http://serebii.net/heartgoldsoulsilver/pokewalker-area.shtml#warmbeach
http://serebii.net/itemdex/firestone.shtml
Chat with our AI personalities
Various.
Castelia City (From man in Pier)
Desert Resort
Dust Clouds (Good Luck with this method)
508 You will need Fire Blow and it will be in between the two rotten logs
You get a chimchar
you see all the Pokemon in platinum and trade from your other games like emerald, blue, fire red etc and that is how you find all Pokemon in platinum.
The Pokemon modifier cheat on the Action Replay or migrate it from Fire Red or Leaf Green to Pal Park on Platinum.
go underground and keep digging till u find a water stone or a fire stone