Possibilities
I would try Emerald Yachts, a brokerage here. They don't specifically advertise charters, but they might either have something available or would be the most likely outfit to know who does. There is no charter service in town that I'm aware of. You're looking pretty much at word of mouth. Another possibility would be Riverside Marine in Racine.You can find both Emerald and Riverside on the web through Google.