Buying a 8spd exa-drive cassette is not an issue... Plenty of stock from both Campag and Miche offerings.
bike-discount.de
bike-components.de
bike24.com
rosebikes.co.uk
rob4bike.com
I don't know why many people think that 8spd campag cassettes are hard to obtain. Almost every German webshop has them, and for us Hungarians, it's also pretty easily obtainable as there are plenty of bike stores in larger towns-cities who keep them in stock.
Miche has more ratio offerings.
IMHO, the two best options are 13-23 and 13-26.