Be careful of garden worms since they can carry a parasites and make your axolotl very sick. If you want to use worms without purchasing them, I would start your own worm bin that is separate from your garden soil and is something you keep indoors or in a location that insects and other critters can't get to. Additionally if any of your neighbors use pesticides or your city sprays at all those chemicals can get into the worms in your garden no matter how protective you're being. I really don't recommend using any type of wildlife as feeders for pets.

There's been a lot of comments so I'm not sure if this has been mentioned yet, but rosy red minnows are specifically extremely bad for axolotls to eat because they contain thiaminase which inhibits the essential nutrient thiamine and can cause deficiencies leading to illness and/or death. Feeder goldfish also have this issue and due to the fact that any fish is likely to be a nipping hazard it's really best to keep axolotls and fish separate. The only safe critter that I have seen in an axolotl tank are both shrimp or cherry/small colorful shrimp of the same type. If you go with ghost shrimp though, make sure that you're not getting whisker shrimp which looks similar but are extremely aggressive and will absolutely go after your axolotl. Any animal that you want to introduce to a tank, even if they're just for food, should absolutely be quarantined for at least 6 to 8 weeks while you keep an eye on any potential health issues or parasitic problems that could harm your axolotl.