I admit it, I'm lucky - I currently live about an hour away from the Fabric District in LA, and was recently just about ten miles down Pico Boulevard from it. So I'm incredibly spoiled - I'm used to being able to find crushed panne velvet for four dollars a yard, or a rich brocade for six dollars a yard. A friend and I once found some faux fur for about fifty dollars a yard - which sounds expensive until I say that the pile was about two and a half inches long, a rich brown color and 60" wide.
If I can't make it down to the Fabric District, I usually shop at my local Joann's. They're almost always having a sale, and I wait until they're having a massive half off discount to buy most of my stash.
Where are your favorite places to get fabric? Anything you just can't resist? I'm a sucker for suedecloth and can't resist buying a few yards off a particularly lovely bolt.