I'll second the Seattle Fabric. If you walk into the store looking to buy a single buckle or strap, good luck resisting all the great stuff they have.
I've gotten some awesome deals on Gortex and Cordura remnants for a couple projects there.
I've actually done what you're looking to do....once. it was a real pain and I wouldn't do it again. Ended up with a fine product and used of many nights IF I were to do it again I would buy all the material abd build it from scratch, would have been easier and way less mess. Be ready to...