The biggest problem with using G3 mags is they do not lock the bolt open on the last shot. They have a reputation for being rugged & reliable in the G3/HK91.

There is also the Rock River Arms LAR8 which uses Fal magazines which can be found for about $15 - $25 for the metric pattern. Inch pattern Fal mags are more. Fal metric magazines work good in the Fal. I have no experience with the inch mags.

The DPMS/KAC/Remington magazines I think are their own design. For awhile, they were very expensive, running over $100. Prices are coming down as they gain in popularity and the MagPul mags even cheaper.

Armalite also makes the Eagle lowers and are legal in Cali. Armalite originally modified exisiting M14 magazines to get around the ban. They make their own which are still based on M14 magazines but with improvements. These are Gen II magazines.

DPMS/KAC pattern magazines and Armalite magazines are being manufactured new. I don't know of anyone making quality new Fal or G3 mags