Cheaper Than Dirt (and I'm sure others) have 20 round USA mags for $19.97. I have several but much prefer the "frankenmags" which I beleive are converted ak mags and were also sold under the USA label. I have 20, 30, and 40 round models and they all function very well. I recently put together a 7.62x39 Oly kit, after resolving firing pin protrusion and extractor issues I ran 2 20s, 2 30s, and 2 40s without any malfunctions. My experience with the new 20 round mags is much less satisfactory.
DanS