eQuilter

I buy a lot of fabric from this site. The fabric is not usually the cheapest if you buy small quantities but what you get for the little additional is worth it. They contribute a portion of their revenues to great causes and you get to choose. They are friendly to changing orders, they seldom oversell (only once and it was more like "we only have 7/8 and not a whole yard left"), they have the best selection of science/geek, asian fabrics - hands down!