Howdy and welcome to the forum. Before answering your question, I've got some to ask......Do you know if the snake is captive bred or wild caught? How big is it? Do you know what species of garter it is? Most garters you'll get here in the states from petstores will usually feed on feeder fish (rosy reds or guppies...depending on the size). If it's a captive bred animal or even if not, it may go over to mice (of an appropriate size) with some work. All my garters are fed mice, which works out very well on a lot of levels. Something else to try would be earth worms (not the "redworms" that are sold for fish bait, actual earthworms or night crawlers, again depending on the size of the snake). Hope this helps.
Roy