I just sold 15 "new" never fired 30 rounders for $8.00 each. I think I may have some more in a box somewhere around here. I don't have any experience with the 20 round ones but the 30 round mags worked ok for me too. If you have any problems with your weapon feeding the first 3-4 rounds just load a few less rounds until they "loosen up" a bit. I prefer USGI or PMags of course but $5 seems like a good deal to me.
I also have a bunch of 30 round Mini-14 Thermold mags. During the ban you couldn't find any reasonably priced Ruger factory mags for the Mini so we shot Thermolds.
If you bump into any old 30 rd Thermolds just be sure they have 22H molded into them. Those are supposed to be the best "pre-ban" ones.