I don't know about the best place, but I Googled "fleece on a roll" and found some websites that way:
https://www.fleece-fabric.com/
http://fleece4sale.com/
http://fleeceforsale.net/cart/index....index&cPath=65
https://www.picotextiles.com/?gclid=...SAAEgKOA_D_BwE
This one is wholesale so maybe does not sell retail:
https://fabricdirect.com/acatalog/fl...id-prints.html
Edit: How many yards are you thinking of? Just looked around a bit on those websites, and fleece seems to run about 60 yards for a roll. The bigger issue may be shipping costs. Depending on what you are thinking of, you may want to consider buying by the bolt instead. I suppose the yardage for a bolt depends on the thickness of the fleece, but not sure. JoAnn's does sell by the bolt.