It is generally not recommended to feed amphibians to captive snakes at all even if they are part of the natural diet. The reason is they tend to contribute to a build up of internal pathogens. Snakes in captivity seem to develop levels of these pathogens that cause health concerns where the levels in wild animals is usually self managed.
Depending on what species of Garter you have I would recommend a rodent diet. If your snake does not take rodents then try scenting the rodent with fish. If that does not work I would use Feeder Rosys (fish) and keep trying scented rodents from time to time. Eventually your snake should convert to eating regularly on unscented f/t mice. This is true for most species of Garters but there are a few that are the exception.
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Randy Hallman
Firehouse Herps