Baines drops it
dropped by Spearslayer
As of this post in version 1.1, I have seen a handful of them in game.
Unable to search for it in our trading post. Most likely not implemented in the game.
Rare item dropped by enemies in Corrupted Breaches.
also looking for this material, already have the other materials
If anyone ever finds it, please share
Dropped this today from Sunburnt spirit in brimstone