Marshall dry goods to the rescue! Only if you have space for a roll though. I purchased a 30 yard roll of hobbs 80/20 last summer for the excellent price of $100 + shipping. That is off their retail site. The roll was not nearly as heavy as I thought it would be, either.
For smaller quilts, I buy white or natural fleece at Walmart. They also have a thin fleece that I like to use in baby quilts because it drapes wonderfully and is very lightweight for the little ones to tote around. Now I'm considering buying some patterned (not plain single color) to use as both batting and backing.