I'll second the recommendation for Henry Boehm. Very nice guy, and from what I can tell, just about everybody in this area goes to him.
Here's a second suggestion: Michelle Fiore, dba Classic Contrabass, Inc. She's in Wheeling, Illinois, just a few miles past the I 94/294 split, far northwest of Chicago. Depending on where you are in SE Wisconsin, she might be pretty close to you. Here' a link to her website:
http://www.allthingsbass.com/