Small earthworms and minnows are probably the most common food. I also fed my Iowa plains garter babies pinky parts. Some people may not like the idea of cutting a pinky into 5 or 10 pieces, but the blood should entice most of them to eat.
I think the most important part about it is to feed them often and get them large enough to eat full sized pinkies. Starting them on pinky parts would mean that they're less likely to refuse mice later on, like my adult female did. She would only eat toads and nothing else for years and never did eat fish or mice.
Good luck