I picked some up from Jensen Arms . Might be worth dropping them an e-mail. I drove up there to get mine, but their site does say they welcome interent sales.
I've tried several different suppliers, and got a bunch of crappy mags. My favorite, and the only ones I'll buy now, come from www.dpmsinc.com. The 20 rounders cost $20, and they don't charge shipping on orders over $25. They are alumnium, feed perfectly, and are the more compact straight 20 rounders (no bend at all). Perfect for shooting prone. DPMS also has a great supply of other parts--I've upgraded to NM sights from them, best price, excellent quality.
rguns has 20 round military issue mags at a resonable price. Now that the "Assault Weapon" law is over and done with, many quality hi capacity mags are showing up at reasonable prices. Some unscrupulous dealers are still trying to charge robbery prices for them though. Pre-Ban and Post-Ban terms are now a thing of the past. Thank Goodness!