You might do better searching with the Rover part number not the Girling one. The Rover part number is 601916
I did a quick search and found eBay seller Voking2000 had been selling them, maybe contact them initially
I cleaned up a rusty pair of piston cups and had them re chromed, they came out well. There is no hydraulic seal to worry about, so they are not a safety critical sizing. With care you can remove the 'guts' in order to get the outer re plated.