SIG magazines specifically for the MPX. According to reviews, they're made by Lancer. A lot of the other pistol caliber carbines are made with AR-15 lowers the manufacturers already have, with an insert for a pistol magazine.

Many of the pistol carbines use Glock magazines, and/or the maker's, if they also make pistols, like Ruger.

I like the SIG MPX magazines for a couple of reasons, not the least of which is that you can load them like rifle magazines - just place on top, and push down. As opposed to pistol magazines where you press down the round at the rear, and slide in.