Brownells carries a Blade-Tech M1A mag carrier. The only problem with it, assuming you are right handed, is it's made for the bullets to be facing front if you wear it on your left side. I used a heat gun to modify mine so the bullets are to the rear.
Ready-Tac also makes a M1A carrier. Check out www.CPWSA.com, you may have to call Chris to order it though.
You could buy just the Blade Tech thigh rig system with all the bolts and straps for around $40.00. Then you could make or have someone make a custom holster for any mag or pistol. I made a rig for my Springfield Operator with a full rail and a lazer/light combo for $150.00 with 2 mag carriers on the thigh rig.