I have found many new favorites from the Quiltshops.com site. If you find some you like, you can sign up for email notifications of sales. Most have sale or clearance sections too. That said, here are my favorites and my first few choices when looking for great quality fabric at decent prices:
Thousands of Bolts -
http://www.thousandsofbolts.com/index.cfm
Quilt Fabric Closeouts -
http://www.quiltfabriccloseouts.com/
Stitch-n-Frame -
http://www.stitch-n-frame.net/
Over the Rainbow -
http://www.overrainbow.com/
Bali Batiks -
http://www.balibatiks.com/
One thing to keep in mind when shopping is that there are many stores now that price their fabric by the 1/2 yard. Also, they do vary from store to store on what the minimum size cut is and shipping can vary greatly. Most places will ship flat rate priority mail, but some go by total price of the order. Just be sure to read carefully before you order.