Mites are species specific. As long as there are snakes around and the husbandry is good(high humidity and temps which mites love), they will generally prefer that type of environment. Rats are kept dry and not 90F. Now it's possible to get them from a feeder but very unlikely. I used to feed my snakes pre killed for years and I never once saw mites on any of my mice or rats.
And for it to happen 3 times? That shop had to be crawling with mites from floor to ceiling.
Anyways, sorry to hear the loss and I would definitely wait at least 2-3 months before I brought a new snake home just so you know that the mites are for sure gone.