We tried to go by an historical mill when we went to the covered bridge festival in Parke County, IN in the fall of 2018, but it was a mob scene. I read about it, and they are a working mill and sell flour. We will try to go back at some point, during a non-tourist time (which is hard because the county promotes tourism year round) and see the mill and buy some of their flour.
We did get to see the mill at Spring Mill State Park, although it was not a time when it would be running. That was where I got the wonderful cornmeal we just finished. We were hoping to go back there this fall, but given the long-term forecast for Covid-19, I don't think we will be doing any traveling for at least a year.
The site that Aaron sent around shows a mill in Grand Rapids, MI. That one would be on my wish list to visit as well.