I rarely buy for a specific project. Usually when I shop on line I am stash building and taking advantage of sales or free shipping. I like Hancocks of Paducah
http://www.hancocks-paducah.com/ a lot but their shipping can be sporadic as well as having things in stock. I like Thousands of bolts, but you can only order full yard increments there. Fabric.com is good but you need to be sure you are ordering quilting cotton as they carry so many different fabrics. There are so many wonderful on line sources.