Mary: Everyone has there favorites, I can only tell you mine. Beware: You can sign-up to receive their e-mails on sales etc. These are my favorites, have ordered from them all with good results:
Fabric.com Now having a sale (4.95 a yard for QS quality)
Connecting Threads Now also having a sale.
Whittles
Thousands of Bolts
Marshall Drygoods (never ordered from them but they have some good deals)
I know there are hundreds more but this is where I go. Maybe others can add others.