I am sure your property is already crawling with garters. You might try improving the habitat like I did but it sounds pretty good already. Plus the government has made it illegal to stock any wild animal anywhere. Although they tax you and me in order to pay for the government sanctioned stocking programs.
You know like they dump trout in every stream. In fact gov't stocking programs are part of the reason for the decline in amphibian populations in some areas. The government put predatory fish in what were fishless lakes. They are stocking yellowstone with wolves at this very moment.
Also if you tried to stock adult garters they may not adapt well to the new environment. My understanding is that they learn where good spots to brumate are and may not make it through the winter if they don't know an area. This may be an old wives tale though since I don't know of any actual studies.
I think the issue of disease is fairly overblown. That seems to me to be a fairly easy thing to control for. I think the main problem is introduction of exotics. The funny thing is that the government has made it illegal to own natives in many places so they have increased the trade in exotics. The rule of the unintended consequence.