Damn,it wont drop from anything 
i’ve searched a bit and found an old post,suggestin the best way to get it by runnin sot and buy from the vendor on 3d floor.Is this still a good advice,or there are news ways to find it?Plus,the diffult game influence in some way the drop?I’m on elite atm,i have to go lower for that run or its the same?
I also searched Inferno recipe for a long time. I had a lot of second tier relics recipes drops until Inferno recipe drops from a random Treasure Trove. I was in Act 4 normal with lvl 50 or something when i finally get it.
Dont worry - it will drop - just hunt for Treasure Troves.