I've bought stuff from
http://questoutfitters.com/. They have a wide selection of DIY supplies, and great service.
The best prices for silnylon I've seen is usually from E-fabric Supplier. Wirerat posted a link here in a previous thread.
Also, you can make your own. Use odorless mineral spirits and clear 100% silicone. Mix in a 4 to 1 ratio. Brush on and scrape off excess.
Directions on Hammockforums.