While I respect your desires to have easy to acquire magazines, there are alternatives. Midway USA shows 3 different mags for the AR in 7.62x39mm.
True, they aren't easy to get. But I'm sure there are other sources.
Part of the beauty of the AR style rifle is the simple detachable magazine. I am looking to build one of these myself. But keep the AR style magazine.
I wish you good luck in setting yours up the way you want it.
NRA Chief Range Safety Officer, Home Firearm Safety and Pistol Instructor
"There are three classes of people: those who see, those who see when they are shown, those who do not see."
Leonardo da Vinci