Were am I most likely to find a drum magazine for my AR and what is the average cost for such a item

The eqipment exchange on this board of course has the best prices.

The best drum is the 100rd beta-c which goes for about 500.

Next is the MWG 90rd which is kind of awkward because it is a "snail drum" and hangs to one side. goes for about 150.

The metal ones go for around 200 and I have heard mixed things about them.

Hope this helps

I can't speak from experienced about any other, but I have had an MWG.  It can be very difficult to load without the guide and pusher.  Even with those, it is still a pain in the drain.  It is bulky awkward, throws the center of gravity off to the left.  But you can kinda support it in the crook of your left arm.  I traded it off years ago for a big pile of USGI 30's.  

All that being said, it WILL feed every round reliably, all 90 of them.

Haven't heard too many positive things about them. I guess you have to lube them with some kind of graphite stuff. Nope, just give me some USGI 30 's.

The MWG 90rd is the poor man's 100rd beta-c mag.