I've got a Regina "Synchro 90" 8-speed screw on cluster (13-21) sitting on my desk here. It's new in the box. I will be getting a Sachs 8-speed 12-21 next week, also new.
I have also got a very lightly used Sachs one (under 100km) available for sale too.
I should be able to dig up some new Suntour 8-speed cassettes and clusters too.
Let me know the ratios you are after, and I will let you know pricing.
(There will also be some shipping costs to UK from Oz though).
Regards,
Chris