Coal mine.

As with many other areas of the game, required items, can be found in most areas of the game, subject to frequent changes, many of which now seem to be even more connected to how much progress you have made, and continue to do,how much in time playing,and real money spent.

So the bank is a good place to look, competitions, and though this doesnt help much,as it's currently closed, I think I've seen it in the seasonal store, I have not kiilled any ubers recently, but have found coal there before.

Definitely in clan store, because I just looked, I think its probably expensive if it's only one piece for 3000 points.