You can try to go in and out of the Deimos city (or Archwing back and forth from the spawn) until one spawns right at the bottom of the stairs. Happened only once for me, but since the spawns seem to be localized, hopefully you'll get one too. Another way to get the new kubrows is just to Archwing in a circle around the edge of the map, quite a few appear on the cliffs.
Did you try doing the new level 6 bounty? I finished 2 stages, went back to Fortuna and it did it for me. Good luck!