Late 90s mtb with shimano ig chain, trouble buying cassette
Every currently produced 7-speed cassette you can buy, which all have Hyperglide-style ramping, will be fine with your KMC Z8.1 and all the other currently produced Shimano, SRAM, and KMC 7/8-speed chains. You should buy whatever appropriate range 7-speed cassette you can get your hands on.
Shimano called out various incompatibilities involving HG vs IG chains. The third-party makers just say their chains work with both. (You can probably find some exception to this if you look back far enough). I've never had an issue dealing with the HG vs IG question by just putting a SRAM or KMC chain on.