94 Rear Brake Conversion

I have a 94 Ram 2500 Diesel that has had the rear brakes converted to Disk brakes. I believe the used the calipers off a 1978 Cadilac Eldorado. One is leaking and I want to replace it but these particular calipers are very rare and not readily available. Does anyone know if there is another caliper that I can use or is there other conversions out there that I can switch to?