I've had great luck with the following:
http://www.quiltshops.com/
Over 200 shops from all over the country. I've ordered from many of them and have never been disappointed in either quality or service. User-friendly search. You can often find the same fabric in several shops but at different prices, so you definitely should shop around. A good place to start a search for something you've run out of. Also great for novelty prints.
http://www.christianlanequilters.cit...ebackings.html
Nothing but extra wide.
http://www.sewbatik.com/
Nothing but batiks, including a decent selection of extra wide.
http://www.fabric.com/
Lots of different kinds of fabrics. Fairly good selection of extra wide. Good weekly sales.
http://www.thousandsofbolts.com/
Quickly becoming a favorite because of prices.