My three top favorites for online fabric shopping:
Fabricmart can be addictive, great quality at superb prices. The other two stores carry higher end textiles and some of the prices will reflect that. They all have fabulous customer service and often run great sales, especially at year end and around holidays. I have purchased many things from these three places and can't remember returning anything because I was unhappy.
Another really popular online shop is EmmaOneSock.com. I have never bought from this site but it is also very highly rated among online fabric shops.