I need a base plate for my rb67 pro s

The camera has not been a current model for about thirty years.

It might be possible to have a base plate machined. Or perhaps 3d printed.

However, a donor camera is probably the most direct way of finding the part because it appears to be an integral part of the camera, not an accessory.

If the camera requires the base plate for ordinary operation, then the camera you have is more of a parts camera than a working example.