Contact your local DNR or fishery department. Sometimes you can buy them directly from the hatcheries. otherwise a google search should give you a list of vendors. US Natives are sometimes more of a pita to get ahold of then tropicals legally.
hmm native minnesotean fish? walleyes, northern pikes...been there once, tried fishing there for days back to back but no luck at all. i even tried at st croix, but hell to no avail.
Check your local bait shop too. Sometimes you find fun little odd balls in with the minnows and the sellers will let you have em for free cause the will eat up the minnow stock. I've gotten Mud Minnows, Perch, sunfish, sticklebacks and even a sculpin that way. Or go smelting. The seine brings up all kinds of cool little devils too!