Thousands of Bolts is good-doesn't do free shipping, but great prices and good service. Pretty prompt, too-I've bought lots there.
Fabric.com, Whittle's, Fabric Shack & Eleanor Burns also have great prices.
Cotton Loft had a great sale-nice stuff for $2.75, but due to the volume, shipping was slow. But the website did say due to the sales volume, expect to take some time. At those prices, it was worth the wait.
And all the fabric has been name brand, good quality stuff so it's well worth it.