www.quiltingcotton.com has great prices. However, if you order 5 yards and they only have 3 yards, for example, then they will send you 3 yards. So, when ordering from them, in the comments section, you need to specify you only want the quantity you ordered.
I also order from www.fabricdepot.com. Their regular prices aren't very good, but, they often have 35% off their quilting cottons. Just wait for a sale.