For muslin, I usually try to buy it by the bolt at Dharma Trading. I think you can also get just what you need by the yard. You can order a sample pack that gives you all the fabrics they sell. It's been a while since I did that, but it really gives you an idea of what they have.
http://www.dharmatrading.com/
It appears to be run by a bunch of hippies (maybe grown up hippies?), but it's great stuff and they are wonderful to work with. I love the Kona cotton, it's not thin and it feels great.