Italo-American is good. You also might try Walles Music, 6846 W. North Ave., Chicago, IL 60635.
There's also Star Concertina and someplace called Snuckes (sp?), both in Chicago either now or in the recent past. Seems like the accordion stores just keep dying. TDA visited LoDuca Bros. in Milwaukee a few years back. They had loads of accordions on hand at that time, but the experience was ... strange. Don't know what they're up to now.
Finally, the guy from AccordionHeaven.com advertises on our site quite regularly. His store, Mahler Music Center, is in St. Paul, Minn., and looks to be quite good. We plan on stopping by next time we're in that part of the country.
Hope that helps.
See ya,
Lewis
P.S. And you could always try Chicago's craiglist.org. We tend to find loads of music gear and other cool stuff on the one in San Francisco.