Board Thread:Skyrim/@comment-11444045-20131109233826/@comment-10207438-20131110005119

it says there's a rare chance for you to find it in chests, but in my 200+ hours with my lvl 84 main character I have only found the one in apocrypha(yes it needs Dragonborn DLC) none in any chests, so I would assume that it is an EXTREMELY RARE DROP.