[anji1971]

Quarantine the wild caught snake for a good 6-8 weeks, and treat the mites during that time. Do not let it come into contact with your captive garter, and wash your hands very well after handling/treating the new snake.
After the quarantine, be absolutely sure the mites are gone before putting the two together. If you can afford it, a vet exam for parasites would help as well, but I know how expensive that can be, so it's just a thought!