Besides the excellent ones already mentioned:
http://www.thousandsofbolts.com
http://www.connectingthreads.com
http://www.clotilde.com
http://www.eleanorburns.com (free shipping thru June 7)
http://www.missouriquiltco.com (neat videos, too)
All of these have been reliable in product and service. There are others, but I can't think of them right now.