[member="sabrina"]
Here's how it works.
Aeten II -- mined out. Mandalorian territory. The Mandos control all of this.
Maramere -- mined out and relocated. OP territory. There was never much here to begin with -- the island deposit was the cargo of a crashed bulk freighter a long, long time ago. OP controls all of this.
Yalara -- mined out and relocated. Fringe territory. used to be covered by a planetary cloaking device; Fringe dug up the remains and used it to justify a few ships that were earned with PC kills. There might be more, but it's pretty much only used for PC-kill stygium submissions, as there was never all THAT much there.
Kelsier -- has natural stygium, tightly controlled. Levantine Sanctum territory. The planet was very recently discovered, and is very secretive, remote, and well-hidden, to the point that not even the Sanctum knows about the stygium yet. The planet was created for story stuff, and getting stygium from it wouldn't make sense at this point.
Basically, the only sensible ways to get stygium these days are to fight OP or the Mandos on a stygium raid, or to kill a PC with over 500 posts, or to capture a cloaked ship and take its cloaking device (assuming it's the same size ship as yours). Cloaking devices are very, very rare.