Muslin is rather inexpensive fabric that you can buy just about anywhere from Walmart to LQS. Now that being said I did not know that there are various thicknesses of muslin when I started up so I bought a package of rather nasty stuff in a craft store and brought it to my crazy quilting class. I was still able to use it as a foundation but from that point on I touch before I buy. If it just for a foundation you can use any light colored fabric because it will not be visible. If you are making a project in which muslin will be visible you can use whatever you choose.