My first stops are:
Thousands of Bolts
http://www.thousandsofbolts.com/index.cfm?&redirect=yes
Quilt Fabric Closeouts
http://www.quiltfabriccloseouts.com/
Desperate Quilters
http://www.desperatequilters.com/
Over the Rainbow
http://www.overrainbow.com/
Stitch-n-Frame
http://stitch-n-frame.net/
I've shopped many others also. If I'm looking for something in particular, I start at
http://www.quiltshops.com/ and compare the prices on that particular fabric.