Well Dragon, I see you're from WA. By far the most common garter you're going to find there would be T. ordinoides. In the wild they are mainly worm/slug specialists. Some will also take fish and pinkies but for the most part I think if your snakes are eating worms they can live long healthy lives doing so.

I actually have a litter of baby ordinoides right now, and being raised in a tank full of baby concinnus (oregon red spotted garters) they have taken to fish and eat it readily. Feeding a WC northwestern is usually pretty simple but they can be picky and only want to eat what they were eating in the wild. You can't really go wrong with night crawlers as all northwesters will eat them.

Worms contain way more calcium than people give them credit for and swear that a diet of worms only is deficient in some way, listing calcium as one of the nutrients lacking but it's simply not true. Worm castings alone (the "poop" inside them) contain enough calcium for snake. A slug, especially with a gut full of greens, also contains significant calcium.

I have kept northwestern garters healthy for years on nothing but worms and slugs but they need to be fed often, every few days, and it can be challenge to provide a continuous supply year 'round.

If you have another species of garter besides northwesterns, any other garter found in WA would need a little more than just worms. In this case, whole, F/T silversides would be a good choice.

I guess the short answer is you can feed your garters worms and silversides, and often salmon or trout from the supermarket is a good idea too. No need for rodents in the fridge or live food at all. Fish and worms is fine.