This is quite a common myth. "mites coming from aspen/orchid bark/mulch/etc"
Snake mites require snake blood to survive. YES it ispossible to bring mites home in your substrate but only if the shop you buy it from has a mite infestation.The mites indirectly comehome in the substrate but they are equally likely to come home on you, your clothes, your hair, any reptiole you buy there etc etc not atually FROM the substrate itself.
The only way to avoid this is to be picky where you buy your substrate from (the state of the animals in a shop has made us switch substrate provider three times. We now have found a friendly mite free local supplier who, like us, treats every incoming animals for mites and keeps on top of it in his shop).
I hear this all the time in the UK:
"oh I wouldn't buy animals from them, they don't kee them very well. we just use them for feeders/substrate/etc"
And I can't imagine it's much different in the USA!
So to sum up - don't buy from grotty shops or shops who think mites are "ok/normal" (Another thing we hear a lot over here)and you can't bring mites home with you in ir on anyhting they sell.