Booo. There must be a stock of NOS 8speed cassettes somewhere.
Whiskers, a roadie shop in Goffs Oak, Hertfordshire often has all sorts of odd gear including mountain bike stuff. They have amazed me in the past when I asked "I don't suppose you have any 7 speed Shimano MTB cassettes in stock" their answer was "which ratios do you want?"
So old school they don't have a website but a Google search turned up their details:
Whiskers (Retail) Competition Road & MTB Parts 684A Goffs Lane, Goffs Oak, Waltham Cross, Herts 01707-875448 TL 323030 Only open 09:30-17:30 Tue & Thu, (cl lunch 13:00-14:00) and 09:00-17:00 Sat, closed Sun
1991 Cannondale SM1000
2004 Specialized Epic Comp Disc