Corsola hold Red Shards, and you have a 5% chance of getting one with a Corsola, however, a Fire Stone isn't required to get a Camerupt, Numel evolves into Camerupt at level 33 without the need for a Fire Stone.
Corsola, Red Shard (5%) > Fire Stone
Clamperl, Blue Shard (5%) > Water Stone
Relicanth, Green Shard (5%) > Leaf Stone
Chinchou, Yellow Shard (5%) > Thunder Stone
http://www.gamefaqs.com/gba/921905-pokemon-emerald-version/answers?qid=37039
You have to look underwater using the HM Dive. In some places, there's small rocks that hold a hidden shard. Once you find a shard, trade it to the hunter guy for a stone.
Route 124; Exchange the Red Shard to the Diving Treasure Hunter / Fiery Path; After getting strength
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You can find a Fire Stone in the Fiery Path after you get the HM for Strength.
find one in the firey path or trade 1 red shard for one at the hunter's house
u gotta find a red shard & give it to Hunter to the left of mossdeep city. He lives on an island with one house on it.
With a Firestone you can evolve into a Eevee into a Flareon, A Vulpix into a Ninetails and a Growlithe into a Artacaine
firestones are for Pokemon who only are fire types