I bought a set from MS Roadrace over in the States! Don't forget about the duty though when they land in the UK, good idea to ask the vendor to write the invoice price down to $100 or similar!! Hope the don't get lost though!!
There are replicas of these seats out there...and they are quite 'whippy'; ie not as stiff as the original shells. Not sure I'd want to be in one in the event of an accident...