I would get about one or two every clear of Mistmoore. Then I went to Sol A and cleared the goblin side of the dungeon and got eight. I'd say Sol A is your best bet.
Found one of these last night. Dropped off one of the Nybright sisters in Lesser Faydark. Cleared the camp a few times and got some nice bronze weapons too.
I looted one of these off of a faerie noble in the world's favorite newbie zone, Greater Faydark. Thought it was going to be another 3 gold loot and blinked twice when the merchant offered me 7 platinum. So, you can add GFay to the list...
I got one of these in Unrest; I believe it was off a regular werebat. As a paladin I've spent more time in Unrest than I can remember and this is the first time I saw it drop. Apparently it's not too common.
After scrolling down this page and seeing that the first post was on June 7th 2002, my guess is that this item was brand new to the game when the new smithing recipes came out since it is stated to be a component of one of the tempers. This might account for the reason I never got one in Unrest until recently.
According to EQTraders.com, the Temper recipe can be brewed as follows:
Firey Temper
-----------------------------------------
2 flame agates in a mortar and pestle = crushed flamed agate
essence of fire + crushed flame agate + imp blood = firey temper