Quiltshops.com
Over the Rainbow
Cotton Capers
The first will let you shop around for the best price.
The other two I just used and they are great but there are others I like too. Keepsake is a great store but their prices are higher. So far I've been able to find what I want at other places.
I quite often shop on line when there is something specific I'm looking for and am very pleased with the results. If you've never seen the Quiltshops.com site before you'll have a lot of fun shopping there. By the way, there's one other thing I should mention. There is a site where you can buy the best fabric at half price from a retired shop. It's JNCQuilts.com (I think the address is right)