You should monitor what's available on German Ebay - both bodywork and seats may come up for sale more frequently than on US Ebay...
Also, R12S seat isn't the most comfortable seat to begin with, regardless of the color, so you may want to find used Corbin (much better, for me that is) or send your red seat to Sargent to reshape and recover it... Or, you can just buy black seat cover from Ebay and replace your red vinyl cover...
