Two thoughts.... One is ebay, I see mirrors on there all the time. Two is DIY plating, there are more than a few telescope makers out there that make their own mirrors. Find one or two and ask if they will replate the mirror that you have.
These mirrors have little variation between models, so just about any mirror made for a Rollei should work. The most you should need to do is change the focus point of the mirror, or maybe change the focus of the viewing lens. Neither one is very difficult. Or send it to one of the often mentioned repair places for a CLA and let them deal with the mirror replacement.