I’m guessing that by “buy their mineral hairball shit” you mean they’re trying to sell you something they make money off of? What does that even have to do with asthma?

Try calling some other vets in your area and just ask about the litter. I bet someone has a recommendation for you that isn’t part of a for-profit plan. Alternately I found a bunch of recommended litters online searching for “asthma litter box”, though I won’t link any here because I can’t recommend anything I don’t have experience with.

What I was trying to find for you isn’t a special litter though. I was wondering if there was a litter box itself that had an air filter built in. Failing that you might try getting a small air purifier like the Honeywell QuietClean (which I do have; it’s just okay) and placing it next to the litter box.