I don't know about Southern Wisconsin ( although I will be following this to find out) but there is a Mercantile Store in McGregor, Iowa right across the Mighty Mississippi from Praire DuChien that handles some guns. Was a fun shop to visit the last time I went through there! Greg.
For purchasing black powder itself, supplies and firearms; I go to Bill's Sporting Goods in Lomira, WI. He's got a selection of long guns and pistols; mostly percussion, on occasion a flintlock. That's where I got my Tulle, 3rd Model Dragoon and Walker.