I know Im kickin a dead thread, but... For any Thamnophis sp. (Garter snakes) Earthworms are honestly you best bet for food form them. If the snakes are small then buy some baby nightcrawlers and cut them into pieces. Also live guppies (not goldfish) are good for garters. Make sure you have ample hiding spots for the snakes also. If they cant hide and are stressed, they wont eat.

Check out http://www.thamnophis.com if you have questions about keeping Garter snakes.