I can get 90" W muslin from the local fabric store for $8/yd. I don't have to pay shipping. I can get it from Jo-Ann Fabrics also. It comes in bleached and unbleached.
I will be using it to cover some back-lit flats.l I figure at 90" W, that is more than enough to cover a 6' W flat. I looked around for muslin at various theatrical suppliers and it usually is a lot wider than I need. It also is a bit more expensive. I don't need "tough" material - this will probably be used for this one show and then be relegated to the scene area after being rolled up.