You can use any .223 mag for 7.62x39, but if you load more than 6 rounds into the mag, the taper of the rounds will cause them to jam up in the mag. The "factory" mag for these rifles was a 20-round .223 mag with a block in it to only allow 5 rounds of 7.62x39 to be loaded.
The ONLY good hi-cap mags for this setup are here below:
[img]photos.ar15.com/ImageGallery/IG_LoadImage.asp?iImageUnq=12317[/img]
The two on the left are G-Techs, and are the best. The third is a Scherer, which is second-best, and the right-most mag is a USA.
All of these will be very difficult to find, and will be expensive. There are no "sources" other than people selling one or two out of their private collections, so you need to watch the Equipment Exchange and the gun auction boards and hope to find one.
BTW, thank Boomer for the pic; I stole it from him. [:)]
-Troy