Drop __cpLocation may be random.
I found a Lens in a regular chest in The Blightbogs on Act III. Ruuko (talk) 03:34, December 11, 2012 (UTC)
I found a Lens as drop from monster (I didn't watch who it was) in Wellspring Temple on Act I. Also I was NG+. AvengerOk ~November 2018
I looted a lens at level 15 before getting to Crow's Pass, so pretty early on. Ruok ~December 2018.
Found one in Avoid the Lava (Phase Beast challenge, NG0) in a blue chest, after 2 hours of farming (about 70 runs) so you can imagine the odds.
Sometimes appear in a chest covered whit a sand cloud in the Ossean Wastes. When you approach, many dune burrowers spawn from the ground. You get to kill them in order to be able to open the chest.