There is a fabric that seems to fallen out of popularity that is similar to muslin. It is named “EcologyCloth”. It is a smooth fabric the color of Unbleached muslin. Handles beatifically and served as a substitute for muslin. Roc-Lon has several qualities and is not expensive.

I buy quality premium muslin from Marshall's. It's slightly seconds and 36" wide but I have not found any bad spots in it at all other then at the selvages. It's bleached or unbleached by the bolt for $.87 yard. I keep several bolts of it all the time.