I frequently by fabric by the bolt. Sometimes I will split a bolt with a frend or relative. It is sooo handy to have the bolt of white or off white , as they are such cornerstones in a stash. Some muslin is on 50 yard bolts so make sure you know how much you can handle both from a volume stand point and $$$.
I have used the LQS for such purchases, as well as online retailers. Do your homework ahead of time and know exactly which one you want... Manufacture , and the manufactures number. That way it makes it easier for your source .