Although muslin is usually assosicated with (and in fact tradtionally refering to) unbleached cotton, it often makes sense to use other fabrics when making test garments, especially when working with stretchy or drapey designs. I like to source my test fabrics in the bargain bins in fabric stores and Ikea is another favorite, since they often have offers on regular cotton and on some other fabrics too. You don't need a ton of different fabrics, but I think every muslin stash should include some cheap knits (both with and without lycra), a drapey fabric (like slinky polyester), and some regular woven cotton.
Here are some examples on what sort of fabrics I've used for test garments.
For my wedding dress I went to Ikea for my muslin fabrics. I knew that they had some cheap chiffon type of fabric, which I needed in order to asses construction issues and drape. The Ikea fabric was a bit stiffer than the silk chiffon I used for the proper dress, but it still worked well.
Regular muslin fabric (ie unbleached cotton) is great for tailored styles, like the rain coat I made a few years back.
When making test garments for knits, I always use knit fabric of similar quality. For this project I just used some odd t-shirt fabric, that I also scribbled on to mark seam lines and details.