Board Thread:Questions and Answers Forum/@comment-27820780-20160218082943/@comment-26355186-20160218173232

I would suggest not farming for them in PoE, as 1: Treasure Keys are rare, and 2: If you would get them in PoE, they would drop with Year One stats, meaning that you would have to spend Legendary Marks and/or a few Exotic shards to be able to use them in King's Fall.

My suggestions for getting them is 1: Waiting for Xûr to sell them or Exotic Leg engrams (could require lots of waiting) 2: Buying Three of Coins from Xûr and then doing lots of Heroic and Nightfall Strikes.