Stick to USGI 20- and 30-rd pre-ban mags. All of the manufacturers are fine except Cooper. Ensure there are no dings, dents, feed lip cracks, or broken floorplates / floorplate tabs.
Check out the mag FAQ at the top of this discussion forum.
Where to buy mags? Check out the "AR15 Magazines" section of the "Equipment Exchange" (click on the link in the top right corner of this page.) Lots of decently priced mags in good shape there.
You can also find them at gun shows, but watch what you're buying. Sometimes they're grossly overpriced ($28.00 for beat-up mags), or crappy (all I could find at the recent Market Hall show in Dallas were dented and dinged mags with feed lip cracks.)
[Edit] Use green followers in USGI 30-rd mags (see the green follower FAQ at the top of this forum.)
HTH,
Gabe