I purchased some from this site: http://www.mybuttonbuddies.com/ButtonBuddiesShop.html
but they were relatively expensive, yet very very easy to make. I did start a thread on the tips and tricks forum about these. And offered a short explanation as to how to make them. Also, I discovered that even cutting circles (with slit) out of flannel, and layering a few together, without sewing. zig–zagging the edges, works fine. In other words, if you can cut out the shape, no need to sew.