A bait shop will probably be a better place to find the appropriate worm.
Buy night crawlers, not pan fish worms. Pan fish worms are redworms/red wriggles/compost worms-all bad for garters.
Most bait shops sell Canadian Night Crawlers.
If you are in the U.K. they may also be called lob worms or dew worms. Someone correct me if I'm wrong.