I'm not sure what your goal is but there are a million of those wheels out there and they're worth almost nothing unless you want something exotic. Check Craigslist, bike co-ops, or, honestly, another 1990s MTB to swap wheels with and re-sell.

If you've only checked bike shops, that's why you can't find anything. A 1990s MTB might as well be a penny farthing to them and they're unlikely to have anything to help other than trying to sell you a new bike.

Oh, and rear spacing should be 135mm.