Snakes mites can be on rodents and they can take a blood meal from them if they need to but they arent going to be the choice host. Probably lice or something like posted above.
Regardless, that is pretty gross and i would definitely find a new feeder supplier.
And if they let their rodents get that infested, i would be worried about picking up a hitch hiker walking around in the place. The hitch hiker being a snake mite.