I've gotten wide backing at Wide Fabrics and have been very satisfied with quality, selection, service, everything.
http://www.widefabrics.com/
Also, Craft Connection is having a sale on wide backing from 11/21 to 11/27, 15% off all in stock backings, if you can wait a bit. It's one of their ever changing weekly sales events.
http://www.craftconn.com/