Fleece can be bought at most craft stores (Hobby Lobby, Michael's, Joann's Fabrics, even many walmarts). For plain colors, it usually just runs a couple dollars per square yard. The benefit of fleece or any other type of fabric, would be that it is washable, so that one would not have to purchase any new pieces for a long while. I used fleece as a cage bottom for along time when I kept rats as pets - replaced it every couple of days and put into an animal load of laundry ever weekend. Less expensive than buying loose substrate, but definitely requires more cost regarding time/maintenance.
Another thing that comes to mind on fabric in general however that I would be worried about, is being able to safely heat an enclosure. Fleece is a great insulator (so many coats/sweatshirts/jackets made of the stuff.. along with other household soft things like blankets and snuggies). While heat would definitely get through the fleece making it possible to create a hot spot on top of the fleece, it would most liekly be remarkably (and dangerously) warm to achieve the top temperature, underneath the fleece. If you measured the head directly on the plastic/base of the tub under the fleece, you run the risk of your snake not climbing under the fleece to be able to regulate his body temp.