Sharping rings are used to bend notes, in the same manner as bottleneck or steel guitar. Nicolas Caballero popularized the sharping ring. He used to use a tuning key for this purpose, and then went to using a ring, so he’d be able to make more use of his left hand for playing.
As far as I know johnkovac.com and paraguayanharps.com are the only online sources for them. I wonder whether there may be some commonly available item that could be used for the same purpose. Maybe from the hardware store. Possibly an S hook of proper size could be worn on the finger and used as a sharping ring.