You can purchase antique rings most anywhere. If you go to an estate sale you most likely will find an antique ring. Antique stores should also have a wide array of antique rings available for your viewing and purchasing needs. If you are really curious, you can even try searching a pawnshop. Jewelers should also have antique rings available. Most antique ring experts suggest purchasing rings from a jeweler who has a good reputation, especially if you don't know much about antique rings.
Antique rings will have a wide range in prices ranging from very reasonable to very expensive. It all depends on the ring.