I'm looking online, and I see several different places to get it. Some say Bonehead, I saw a video of the Warrior dropping it, it says that you can find one in any lootable chest...I don't see anything concrete. I also see that Hyperius the Invincible drops one. So which is it? --- I have Dysarthria. It's a Speech problem. Your question is answered now.
I have received three from Pyro Pete...that has seemed the best source so far that I've witnessed. bone head does not have any higher chance, that was a rumor started by a type-o; gearbox debunked it on their forum awhile ago.