I live in Milwaukee, which is about an hour north of Chicago. I go to Mitsuwa once or twice a year, and the bookstore in there is called Sanseido. They carry a pretty huge selection of Jpop, but I really would not buy it. It is HORRIBLY over priced and they'll rip you off. That's where I bought my first Ayu single, Moments for $40.
Don't do it. There's got to be better places to buy.
BTW, the website for the Chicago Mitsuwa is
http://www.mitsuwa.com/tenpo/cica/eindex.html. Also, they're not actually in Chicago, it's in Arlington Heights, about 30 minutes northwest of Chicago.
I would suggest going to an anime store, since they often sell Jpop.