You can use any capacity mags that you can find, up to and including a 100rd drum. Fortunately, 30-rounders for the AR were so plentiful before the '94 ban that their value has not gone up appreciably. You can find good quality mags for $15-20.
A pre-ban can only be purchased from a dealer or individual. Manufacturers do not keep old pre-94 lowers in stock. Some good sources for pre-bans:
http://www.ar15.com/
http://www.gunsamerica.com/
and, of course, the classifieds on our own fine TFL. You can expect to pay $1200-1600 for a pre-ban. I have not seen too many ads for lowers only. Usually they come as a complete package. But you can always buy another pre-ban configured lower if you have a pre-ban upper. There are a whole lot of other rules that have to be followed with pre-bans, but suffice it to say that if the receiver left the factory in a pre-ban configuration before Sept '94 (i.e., with all the evil features), you can dress it up just about anyway you want. If it left the factory prior to Sept '94 but did not have some evil features, you cannot add them on now, even if it is pre-ban.
Finally, if you think this maze of confusing laws is punitive and unfair, join the group. But instead of just complaining about it, get get involved politically. Join the NRA, join the GOA, and write and visit your congresspeople.
A pre-ban can only be purchased from a dealer or individual. Manufacturers do not keep old pre-94 lowers in stock. Some good sources for pre-bans:
http://www.ar15.com/
http://www.gunsamerica.com/
and, of course, the classifieds on our own fine TFL. You can expect to pay $1200-1600 for a pre-ban. I have not seen too many ads for lowers only. Usually they come as a complete package. But you can always buy another pre-ban configured lower if you have a pre-ban upper. There are a whole lot of other rules that have to be followed with pre-bans, but suffice it to say that if the receiver left the factory in a pre-ban configuration before Sept '94 (i.e., with all the evil features), you can dress it up just about anyway you want. If it left the factory prior to Sept '94 but did not have some evil features, you cannot add them on now, even if it is pre-ban.
Finally, if you think this maze of confusing laws is punitive and unfair, join the group. But instead of just complaining about it, get get involved politically. Join the NRA, join the GOA, and write and visit your congresspeople.