Stitches 'N' Things is another good site. I'm in Australia & have dealt with them for over 15 years. They are a real bricks & mortar shop as well as a website.
Fat Quarters are also a measurement of fabric used in Cross Stitch. There is no difference in quality to that on the bolt. It can be in Aida, even-weave or linen. There are different thread counts (number of threads per inch) in all fabrics.