I'm not sure about garlic. I know garlic is good for fish, at least some, but I don't know how it would affect an axolotl. I think technically any bait shop will work for getting worms, and I believe any earth worm will do. As for additives, I know some people have found worms that have some kind of green coloring to them, it's supposed to attract fish while fishing.
Red wrigglers do better in our climate here in US, while canadian night crawlers are a colder climate worm, so it all depends. If you want to start a compost or small worm farm, red wrigglers. If you just want a worm to buy and keep in your fridge, either works but canadian night crawlers are more readily accepted. Good luck!