You know that these magazines do NOT hold 20 rounds of 7.62x39, right?  They are USGI .223 20-round mags with a U-shaped block inside to block the mag to 5 rounds of 7.62x39.  They work just fine, but then, any .223 mag will work with up to 6-7 rounds of 7.62x39.  Anymore than that, and the rounds will jam up in the mag.

There were never any factory hi-caps for 7.62x39 ARs.  The only working aftermarket hi-caps are the hybrid (half AR, half AK) mags, and the only place to buy these is out of other people's collections.  They typically run $75-150 each.

-Troy