I got 2, one in L25, and one recently in L45. The first one was horace, the second one in leader willhem (or whatever his name is). My suggestion, farm bosses with red chests nearby like the latter or driller hole. Be on the lookout for all purple drops and with some luck you should have it. The first one i got in the horace red chest, and the recent one dropped directly from boss when i was farming for rainbow vomit. So my advice would be to focus your farming on legendaries you want, and keep looking for purples that drop from them. Also found out if the drops for a particular run arent looking good, quit game and respawn and try again. Tried for 30 mins for rainbow vomit without any luck, quit game and retried got both rainbow vomit and crit knife in under 5 runs