New York: Manhattan Avenue around Nassau Street in the Greenpoint Section of Brooklyn. Lots of Polish delis, beauty shops, bakeries and Stylowa, arguably the best Polish Restaurant in the city.
Chicago: Milwaukee Avenue, from Belmont NW to Lawrence (and nearby at the intersection of Belmont and Central) the heart of the largest Polish community in America with Polish stores of every kind (little English spoken). Check out the "Polish Barnes & Noble," Polonica on Milwaukee near Lawrence for the largest selection of Polish literature in the world.
If you are on the south west side of Chicago, check out the large community of "mountain Poles" around Archer and Pulaski, not far from Midway Airport. Great bakeries and sausage shops.