Super Sport,
You are correct, USGI magazines are the way to go. Unfortunately, due to the ban on High Capacity magazines, there are no more available to the general public than what was made before 1994.
On occasion you will find NIW (New in Wrapper) USGI's that have been stored for years and discovered. Many times however, the exterior Moly finish is rubbed off pretty bad.
One thing to look for are 30 round USGI mags that have the "Green Follower" installed. This is an upgrade that helps to insure the last 2-3 rounds don't "Nose Dive" into the feed ramp because the original "Black Follower" design "Tilts" forward.
There has been a lot of activity regarding "Refurbished" USGI Mags this past year with the discovery of "Perma-Slik G" which is the Mil. Spec Moly Coating used on USGI Mags.
2 coats of Perma-Slik G, baked on in the oven, a new spring and a "Green Follower" on the 30 round GI mags. (the original alloy follower on the 20 round mags needs no alteration or replacement)and you're as good as new. Maybe better. These sell for $20 to $25.00 on the "Equipment Exchange Forum" on this web site.
Some of the the contractors for USGI's were Parsons, Sanchez, Adventureline, OKAY, and Colt. They are all of the same quality because they all had to conform to the military spec.
Email me for reliable and reputable sources of USGI mags at reasonable prices.
Good luck,
ACK