Where would you find stuff like this?
It is more needed for a Halloween AND a project. One is a LotR project where I'm a Nazgul, and then I'm selling it to some guy for a lot of money.

Would an art store suffice? I'm looking for quality if possible.

Do you mean on the internet? Ebay (http://cgi.ebay.com/KAUFMAN-KONA-Black-Cotton-Solid-Fabric_W0QQitemZ110298566384QQcmdZViewItem?hash=it em110298566384&_trkparms=72%3A1205|39%3A1|66%3A2|65%3A12|240%3A13 18&_trksid=p3286.c0.m14) is the only thing I can come up with, that link is pretty weak although i'm sure you could find some other things on there.

There are sure to be some costumes shops around your area, that will be your best bet, you'll probably get some good advice there, too. You've leaft it a bit late is it not this weekend? Better get out and look around soonish. What is 'the project'? Yes, an art store should do. Anything would do if you're only going to be wearing it once. Just as long as its not too cheap - you pay for what you get.

Who's going to buy a costume from a guy that does'nt know where to get material from

A fabric store would be your best bet. You can get yards and yards of fabric if you need it, and if they don't have what you're looking for they will likely be able to order it for you.