It's not too difficult to get a plains garter to expand their food preference. I found it's pretty easy to slowly introduce new foods with a few tricks and get them to take other things in the long term.

for example, a plains garter I have now was only fed tiny live fish. I got him to eat silversides fish meat. Then I started mixing in pinky parts and night crawlers.

That snake will eat anything now, without any special tricks.

Try chopping up some worms and mixing in chopped pinkies. Over several feedings, slowly back off of worms and feed more pinkies. Basically you have to transition them over gradually.

Also, try some trout, salmon, silversides, or other "safe" fish. Most plains garters will eat fish. If they'll eat fish, they'll eat anything that smells like fish. Even if it's just pinky parts rubbed with fish smell at first.