A couple of options come to mind, the most immediate being the G.I. issue M1A1 folding stock carbine *holster* used by some paratroopers but more commonly by glider-borne airborne units. The originals are pricy and worn by now, more in the category of collectors items, but quality repros are available, or a craftsman could recreate the design in leather or Kydex.
Second choice is an M60 machinegun spare barrel bag, available for as little as $10 in used condition. This is more of a case than a holster, but since it comes set up for an over-the-shoulder carry strap, it might be usable.
Third option: a tennis racket case. I've used this one for a folding stock Remington 870 shotgun with about the same barrel length as an M1A1 carbine, and it's okay if a bit slow. Again, more of a case than a holster, but very credible if the handle of a sawn-off thrift shop tennis racket is added to stick out the end.
BTW: about the use of the M1 carbine, and its alleged lack of stopping power, which ballistically is pretty close to a fairly hot .357 magnum load. [.308 110-grain projectile @ 2000 FPS for around 1000 foot/pounds energy vs. .357 125-grain projectile @ 1500 FPS for circa 650 foot/pounds] I've been involved with three shootings with the M1 carbine, and in two of them the carbine resuklted in one-shot incapication of the target; in the third, multiple shots were fired into the occupants of an automobile and one died immediately while the driver survived long enough to crash the vehicle, then lose consciousness. The carbine can get the job done, particularly if hollowpoints are used, the barrel remains the full 18-inches length, and the targets are hit in vital areas, ie, center of mass/center body or headshots.
So far as those hollowpoints go, I've mostly used the Winchester version, but I believe either Mag-tech or Cor-Bon also offers one, and Hornady has an offering from Tom Byrzynski, designer of Remington's Golden Sabre and Winchester's Black Talon JHP bullets, that ought to be pretty good but is not yet in wide circulation. I also like to add a couple of .30 carbine tracers in the mag to let me know when I'm down to my last 5 rounds.
The 15-round magazine is the preferred mag for no-hangup use from a case or inside a vehicle, but having a 30-round mag handy for the first reload is certainly okay, and having a pair taped together is particularly thoughtful, if a little heavy.
