If you've purchased anything from a store that has snakes, that's a way to get them. If their snakes have mites, they can crawl around the store and get on the merchandise. I'm assuming you probably bought her food from a pet store? Rats can have their own species of mites, which actually don't affect snakes, but they CAN carry snake mites if someone touches a snake, then touches the rat.

I haven't seen anyone use that Mite Off stuff with success. I personally use the RID lice bedding spray from Walmart to treat cages. I wash the cage with soap and bleach and rinse several times, then spray a very light mist all over, not enough to actually form droplets. I let dry, then rinse the cage with clean water (no soap or anything) and dry with paper towels. Then I wash the snake with dish soap and dry them off with paper towels. I've had two snakes with very heavy mite infestations, and doing the cage once a week and washing the snake every other day took care of it. One of the snakes it took about 3 weeks to be certain they were gone; with the other, I think it was only 2 weeks.