30/11/19: possible for your save to be bugged and no bounty hunters spawn, there is a fix in the works according to one of the comments so you can either wait for the fix, or start a new save file (proven workaround in the comments)
There are three types of Bounty Hunter.
Haxion Brood Bounty Hunter
Haxion Brood Bounty Droid
Haxion Brood Commando
they spawn at random (sometimes in mixed/un-mixed pairs), and in seemingly random locations (seem to respawn upon return to planet).
on Kashyyyk they spawn in several of the rooms in the Imperial Refinery, and the Shadowlands.
on Bogano they seem to spawn in the Hermit's Abode.
on Dathomir they can spawn in front of the Temple, in the Temple and at the Meditation Point just prior to the Temple.
on Zeffo they can spawn at Broken Wing (mythical creature room), a bunch of areas in the Venator Wreckage, the Warehouse at the Landing Zone, and the Village.
These are just locations of the top of my head, there are probably more.
thanks Tenreth for pointing some stuff out.