Three Rivers sells them. I believe the "Eastern Archery" site markets them, also.
There are a few threads on making your own thumbring on the Spitfire site, also.
I have good luck using ABS drain/waste/vent pipe. I used the collar from a 4" elbow. The inside of the collar matching the curvature of my thumb quite well.
Drill a hole, and then file hole to fit your thumb snugly. File the "tip" of the ring to match the profile of the thumb, and thin the edges down so as not to abrade the string. Hollow out the inside of the ring, where it rides on the ball of the thumb. A little experimentation will get you a ring that works well, and doesn't chew the h*ll out of your thumb!