Milestones and strikes from what I have experienced

Nearly every one I've gotten have come from standard purple engram drops; either from enemies killed, strike chest rewards, or public event/well grinding. Not to say that these are great sources, I think I've had around fifteen of them drop since Forsaken launched, but from what I've seen they drop primarily from the standard legendary engram pool so your best bet would probably be to use one of the current engram farms that rely on killing enemies to get engram drops. From what I've seen, you're not likely to see one from faction engrams or planet/activity based powerful engrams like crucible or Dreaming City powerful engrams. I will say, I got a god tier Mk 44 with outlaw and rampage, and after testing it out a bit I do like it a lot for pve. I just wish cores weren't so scarce so I could infuse it to 600 and masterwork it.