I wouldnt buy substrate from a store with an infestation either.
Just like you, I walked into a local pet store that has a relatively large reptile collection. I was looking over the snakes for sale, and noticed an aquarium with spider bp's, and baby boas housed together. Obviously not a good combination. I noticed mites on the spider BP's, but not the boas.
Its clear that mites can hitch a ride on your clothes to your car, or even worse, your house.
So this got me thinking, there's a good chance the substrate is infested as well.
Then i realized this could be the case at any pet store that sells substrate. What can a snake owner do to make sure that you dont contaminate your collection with infested substrate? Would freezing the substrate kill the mites? Is it safer to buy substrate from a supplier that doesnt sell snakes too?