Where can I find (undying bear leather)?

The Undying Bear is part of one of the Way of the Hunter quests that you get from the outposts' bulletin boards.

When you take over outposts, there will be hunting missions available in the bulletin boards close to the outpost. It will show up as blue dear head on your map. Those are hunting missions and as you do these missions, you will eventually come to the Undying bear Hunt.

No answer here, because Julie Su has it right, and for once, somebody gives us a straight answer, instead of telling us that we should just "Go do the hunting missions and you'll find it eventually", that there, is not an actual answer, it's just blubbering jibber jabber, that wastes everybody's time.

Thank you very much Julie Su for giving everyone a straight answer, I'm sorry if that sounded sarcastic, but it's not meat to.