It depends on the species and locale. I've got three wc garters from CA.
I have a T. elegans from Humbolt County that I collected as a small juvie. It refused unscented pinkies the first few times I offered them. After scenting it with a Pacific treefrog it took a pink, and has not demanded scented pinks since.
I have another T.elegans from Santa Barbara County, it was collected as a neonate and ate the first pinkie I offered it unscented. Several years ago, I had collected another T.elegans from this location for my college's herp room. This snake also never required scenting.
My third garter is a T. sirtalis from Santa Barbara County. It was my first garter and was collected as a youngster who looked like it hadn't had too many if any meals before it went into brumation for the winter. I've had it for almost a year now, and it will only eat pinkies if they're scented with Pacific treefrog. I've tried weaning it off the scented to no avail.
All my garters will take and have always taken feeder fish (guppies, mosquito and gold fish) without hesitation.
Over the years I have kept wc adult T. sirtalis for brief periods. These refused fish, but fed readily on worms and pacific treefrogs (they were taken from areas without fish, and they weren't about to try anything new). I've also observed wild T. sirtalis gorging themselves on western toads.
At work, we've had T.elegans occaisionally raid our pitfall traps where they've taken harvest mice, shrews, and western fence lizards.
If you're planning on catching a garter, catch the smallest one you can find; they seem to be the most flexible in terms of diet. If you want one that will feed readily on mice, go with T. elegans.
-Alice