They probably can't find your converter because it is almost 40 years old and is not longer made, and technologies have changed for DC power supplies, using solid state electronics for reduction in size, weight, cost.
You (or the RV dealer) should be able to find a modern replacement of appropriate size, although some fabrication might be needed to install it in the same place. It might even be reasonable to replace the old power distribution box with a modern unit, if that was integrated with the old converter.
Online supplier, I would go to Best, or PPL Motorhomes. But most likely, I would discuss the need with the head of service and parts at my local RV dealer, and let them find a replacement through their wholesalers.
Tom Test
Itasca Spirit 29B