Belle Isle?
Probably Easterns, with the red they have on their necks.
We also have Butlers Garters, Ribbon snakes and Dekayi here in Michigan.
I'm In Mount Clemens- Probably not that far from you if you and your friends visit Belle Isle. (I have one eastern, a checkered and a number of transient Butlers.)

Michigan Easterns are sometimes feisty when they get larger, but bites are seldom more than a sligtly bloody scrape. (They have tiny little needle teeth hidden in their gumline- think 'Toothless' from 'How to train your dragon'.)Handle them regularly to keep them used to people and they should calm down again.

Great Lakes reptile, on Gratiot, around 8 mile is a pretty good store- they do handle some garters tho they know more about the more common pet species.
'The road show' is a decent place for reptiles and supplies, but they don't know garters.

The advice previously offered about feeding is good- scent what you want to feed by mixing or dipping it in something they are familiar with- most snakes will come around eventually. My self caught eastern took readily to worms and fish, but pinky mice took some convincing at first. Make sure the mice aren't any bigger around than the largest part of your snakes body- maybe a little smaller at first. My Eastern was reluctant to eat anything that was much larger than the worms and minnows she was used to in the wild.

I only get on the forum once a week or so, but if you'd like somebody local to call and ask questions of, send me a pm and I'll get back to you when I can.