I haven't done a lot of online fabric shopping but recently got some great fabric from Fabric Shack. It was to match some fat quarters that I picked up a while back and could never find anything to go with it. They had the same line of fabric at a good price so now I have what I need for borders and backing. Choosing specific colors is a problem unless you're buying from a matching line.
I've got some good stuff on Ebay but also some not so good. You never know. I've got jelly rolls and charm squares but you can't really tell what you're getting even pattern and colorwise because the pictures are a "representation of the type of fabrics" included. They'll work for the scrappy quilts I've planned. Now I'm sticking to stores that are recommended on the various forums I frequent.