One place that seems to have some inexpensive quilting fabric is
4my3boyz.com. I used them a lot when I used to swap I Spy squares.
Hobby Lobby has a decent selection of boys fabrics. They have a 40% off sale on fabric fairly regularly. You can check their ad online. It's a great time to get backing fabrics. Their regular price is $12.99 per yard for 108" fabric. If I calculated correctly, it's $8.80 per yard on sale. It can often be used as a blender. They also have white, which my guild is always running out of. So, it's a good time to pick up a yard or two to donate.
I never seem to buy there, but Marshall's Dy goods has name brand fabric for $7.99 per yard (I'm on their e-mail list). There is usually a good selection of I Spy type fabric. Also, it's been reported here that their own fabric is good quality. It's usually really inexpensive compared to retail.
bkay