I don’t have much experience buying thumb rings. I bought two from Vermil and their quality is good.
Without a long story, I ended up making my own because the MR (middle) joint of my thumb doesn’t bend.
So far, have made them out of wood only - scraps left over from other bow builds. Started out with Manchu rings, the tubes in the photo. They work fine with bows 48” and up, but with a really short bow, like 36” braced, the string is inclined to jump off the ring unpredictably during the draw. So I made some Turkish rings that would work with my stiff thumb. They are front row in the photo and made out of Purpleheart. If you are right handed, you will shoot off the right side of the riser. Took me awhile to get used to that bit.