My favorite place to go, of course, is JoAnn Fabric. Not only does it have lots of variety in fabric, it has rows of fat quarters. I can make 3-4 books out of a fat quarter, depending on the design of the fabric. Sometimes Hobby Lobby has a good bin of fat quarter bundles too.
I do love recycling fabrics as well. Right now I have five books listed that I made using just one pair of my husband's ABU pants. As I mentioned above, there's a purse that I picked up at a thrift store that I'm taking apart and trying to figure out how to make a book out of it.
By looking at my books, however, I see that one of the biggest sources of fabric that I used was my friend Heather, who taught me how to make books by hand. She had a bin full of fabric that we loved to just open up and let ourselves dream of all the things we could do. If I hadn't moved to Nebraska and she hadn't moved to Japan, my Etsy shop would probably have two proprietors. ;)