First mission in Julius the Troll's story arc is Igneous; the rest are only trolls. Also, the last arc from Talshak the Mystic has them as well as trolls.
Only other sources are the trial and the occasional one that pops up in the Gulch.
I find your lack of signature disturbing.
Drat - I was gonna invalidate that topic since I found the info on a previous thread (my search-fu was off by a typo in Vlocanic. :-p)
I saw that people advocate teaming to get them to spawn in the hollows' outdoor maps. If I crank up my notoriety settings to x8, does that work for the Julius mission? I'd hate to get 3 people bored silly just to help my level-50 kill grey stuff. :-p
I found a low level who wanted the badge, then I would jump around till I saw the smokey areas on the ground in the gulch, let them be the leader so my level dropped, then when they spawned, I was made the leader and dropped them quick. Clumsy as heck, but it got me the badge after a bunch of runs like that.
Le Blanc 50 Dark/Dark Scrap
High Huntress 50 Archery/NRG Blast
And a goatload of others. On a goatload of servers.
Official Rickroller of Hero Con 1
My experience from getting this is that you really want two things: a team and an attack power that doesn't require a target.
The Hollows spawn sizes are based on team size and unfortunately do not seem to scale with your difficulty. Having a team of four or more makes a LOT more bosses spawn (incidentally, the other team members don't even need to be in zone, they just need to be on the team).
Having an attack of some sort that doesn't require a target (such as a patch power or a PBAoE power) allows you to attack them while they are undergroud without having to use Le Blanc's method of finding a lowbie.
Thanks for the replies guys, now I know what I gotta do. 
Wow, the difference was amazing! Even with only a group of three, getting the 92 Magmites I was missing took only 30 minutes, which is half what finding the first 8 took me on my own - so essentially it went twenty-three times faster!
Thanks for the tips you guys gave me, it saved what's left of my sanity !
Is there a good mission/arc (aside from the Cavern trial) to hunt Magmite Lords? I'm lucky if I find 4 an hour combing the center of the hollows, and I never saw one in the Trolls Tunnels.