How do you get firestone in Pokemon Emerald?

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

http://answers.Yahoo.com/question/index?qid=20080314084937AAbpACR