I've not had any reason to look for "Hypoaspsis mites" but that seems to be a fairly expensive way to control Fungus Gnats.

Well, they can be used for other things. But they're seldom successful in the home garden for various reasons.

So, what is it that you're trying to manage? Perhaps we can help with a different strategy.

I'm trying to find a permanent solution to the fungus gnats in my house plants, and I figure that hypoaspsis will be the solution.

Instead, use Bacillus thuringiensis israelensis, -- the abbreviation is Bti.

It's in products used against mosquito larvae. Sold in several products, one of which is Mosquito Dunks.

You add the Bti to the water you use for the plant. It will take 3 weeks to disrupt the lifecycle.

Overall, cheaper and more effective than the mites.

Keep the top 1 inch of soil dry. Water from the bottom will reduce moisture in the top inch of soil.