My favorite place to buy felt is Hobby Lobby. I've never actually been to the store because it's prohibitively far from me, but their online site is very easy to use and navigate. Just look at the variety of felt they have! This is my stash of felt I currently have. I was cleaning my room and decided to show you :)
Craft felt is so inexpensive and versatile. It is my very favorite craft supply to have on hand for things. While quiet books are my biggest use of felt, I also have made beanie babies, decorated t-shirts, and made felt food.