Here is a picture I found from a grocer in Prairie Du Chien:
https://dynamic-media-cdn.tripadvisor.com/media/photo-o/11/f...
Here is an article I found about it:
https://www.outdoorlife.com/blogs/cast-iron-chef/ritters-fis...
I am not super familiar with anything south of Dubuque, but Highway 35 between Prairie du Chien and ending with Lacrosse Wi is full of a number of small river towns that sell it (or at least did when I was growing up there). De Soto and Genoa are the two that had a lot of river catfish and carp, both smoked and sitting in grocer's fridge units.
I will have to pick up some smoked carp if I make it to said area. I've never seen it for sale. The only time I've seen Carp for sale period is in Asian supermarkets.