I have a Norinco Chinese SKS and am looking for a source for reliable mags. I recently ordered a 20 round Blued Steel Mega Mags from www.gunaccessories.com and they stop feeding after about 3 or 4 rounds. The 10 round that comes with it feeds fine.
Get the Tapco 20-round polymer mags. They have worked flawlessly in two SKS's of mine. They are less than $20. Cheaper Than Dirt sells them, or direct from TAPCO. If you still have your wooden stock, you may have to shave a little inside the mag opening.
you may not want to hear this, but... keep the stock one! It works fine, and you can reload the sks pretty fast. Better to avoid ftf's and reload more often, than to have a larger capacity that may or may not work.
Tapco is the best for sks mags. but best using the 10 round fix'd with stripper clips ... I know after spending a bunch of money on clips that fail all the time, tapco works the best. 20 rd'rs are tough to find at the moment, a lot of places sold out of them or on back order
also if you or anyone can get your hands on a fix'd 20rd'r, get it. I have one on 1 of my chinese sks's, came with the gun when we bought it. Works great as well and it takes stripper clips!!!