[BMWCSRegistry] Rear brake piston/caliper

[ulfhjelm at comcast.net]

I seem to have a badly pitted left rear brake piston on my 72 CS.
My mechanic has not been able to locate a replacement part, but he can get re-built brake calipers (@ 250 each) - which seems to indicate that someone out there has pistons so they can rebuild the calipers.
 
Any idea of where I can find the piston (or a cheaper caliper?) 
 
Thanks,
 
Ulf