It goes pretty quickly on the fist of Crota mission. You don't even have to fight the fallen and hive where you first spawn. Head to the left where you get underneath the buildind's second story alcove. Summon your sparrow and go as fast as you can past Omnigul avoiding the Knights she spawns after she screams. Once you are safely inside proceed till you see acolytes summoning the blade. Jump up onto the platform on the left. It'll give you the high ground. There is even a crate you can stand on if your guardian is low enough level that surviving a melee attack from him is iffy. That way you can blast away at him (careful of his acolytes though, they'll be shooting at you).

They took them out....quite a few threads on this already and, I believe they mentioned it in the recent weekly update.

Run the Fist of Crota mission. There is one that spawns inside the building just past Omnigul. Rinse and repeat to get 3. Bungie suggested this as well as they are no longer appearing during patrol.

Do The First Mission from Eris called Fist of Crota. They are in there.