Look at my hyperlink in reply #11. I'm 95% certain that, there is the item you need. You'll have to buy the whole cassette. If you type "Suntour cassette" into ebay several come up but that was the least expensive 8 speed one that will fit your requirements. You might want to act fast because there were only 2 left. I think another vendor had some but they were $75 each.
Suntour went out of business years ago. Back then the manufacturers were in a war to create the best indexing systems and Suntour lost everything. They're high quality parts. They're just not Shimano compatible. Suntour had their own cassette spline pattern (the first accushift-AP, isn't even compatible with yours- APII). Suntour cog spacing is unique to them as well. The smallest 2 cogs are like 0.2 mm wider spaced than the largest cogs - don't ask me why, it has something to do with the pull on the shift lever combined with the arc of the rear derailleur. It works alright if you use all the matching parts. If you're not willing to conform to this construct, this might be a good time to get a new drivetrain. Shimano stuff works better, is more uniform in operation, is more available, easier to set up, more customizable, blah, blah, blah. Honestly, I would have thought that other Suntour "experts" would have attempted to educate thee by now. Personally I would advise that if your other parts are sufficient, stay with the Suntour. You've got something unique that's high quality and will probably last another 10 years easily. Go and get that cassette, and continue educating yourself on bike mechanics from the helpful folks here.