Fabric.com, especially when you have a discount code
Connecting Threads (they frequently have discounts, too)
Hancocks of Paducah (their catalog is GORGEOUS!)
I just placed my first order with J Hittle, his prices are really good, plus free shipping with $100 purchase. I just ordered (and received today) the Ironslide 2000 from Joann, and wish I had waited and ordered from J Hittle, it's cheaper. He has the Supreme Free Motion Super Slider for $18.90, Machingers for $5.08, etc.
I've looked at Thousands of Bolts, but the pictures and colors are so hard to see.