My garters are fed as follows:

Any who willingly take or can be tricked to take rodents are fed a steady diet of them. They are supplemented twice a month with worms and fish. I also use reptivite occasionally.

As far as worms go, you want to use standard earthworms or nightcrawlers that you would buy as fishing bait. I buy canadian nightcrawlers. The walmart in my area supplies them year round.

If they are red in color or have visible color rings such as those little red worms that are used to bait trout, do not use them. These worms are dubbed by many to be poisonous to garters and do more harm than good. The fact that they secrete a nasty smelling orange slime should be enough to deter you. They are tempting to owners of baby garters. Instead, dig up some small earthworms from your garden (assuming you don't use pesticides) or chop up nightcrawlers. To make them easier to chop up, run them under semi hot water, it kills the worm, Yes I know its mean but it's practical as it prevents you from having to chop him to bits while he's alive.

Fish, are tricky. You have to be careful because of the damaging efects of thiaminase. Still, there is no clear complete list on which fish contain thiaminase and which do not. What I can tell you is do not use feeder goldfish. They contain thiaminase. I prefer feeder guppies and rosy reds. My babies all grow up on this diet until they are big enough to eat pinks.

Also, never feed garters fish that has been frozen/thawed, and do not boil or cook fish prior to feeding. This will destroy much of the nutritional content of the meat. I know some people like to use slices of larger fish. I do not do this therefore I cannot tell you which large fish are safe for yuor garters to eat.

I have heard of people serving their garters red meat as well. I personally don't find it necessary and I don't think it's really natural for garters to be eating beef, but I don't see where there would be any ill effects if fed as a treat from time to time.