oh man, if you only played WoW recently... I think they borrowed Destiny's system in the newest xpac (they even put in a Destiny easter egg, but i digress): "They can drop off anything, from almost anywhere, but harder content is where the best chances are." So my guess, and just a guess, is you just need to plug away at it and get lucky. Do the most top-teir stuff you can do, and hope. Any other time, just loot everything you see.
i only say so cuz ive only played for about a week now (im that new), and 1 have like, 10 exotics already. Crimson, one of them. But the last exotic i found; i think i got it from just a random cache on mercury. No elite kill or nothin'. Just a random treasure chest to find. Same thing happened in WoW for me: random chest with usually so little inside, POW! Jackpot!
basically, a Dreg could drop one for you (though it's the least likely place, it may be possible, idk for sure), you wont know until you kill it.