www.avionix.com is the website for Eastern. They have at least one RMI indicator on their used/reconditioned listing. They are very knowledgeable, and can help.
I had a fit when I got my plane back from Pitot/Static, and the ADF didn't work. They fixed it immediately, a small thing, but many approaches have an NDB as the IAF, and there are still places in this part of the world where thereis only an NDB approach.