http://www.owfinc.com/ has fairly cheap material, I use this the most
http://www.questoutfitters.com/ is similar but I've never used it, just looked at their site
http://thru-hiker.com/materials/index.php has some state of the art materials at a little high prices, never used them though
http://www.seattlefabrics.com/ a little more expensive but they have some stuff that others don't, like polyester spinnaker fabric
just noticed you're in Western Washington, Seattle Fabric has a nice store in Seattle.