Saginaw is to the north, Lansing is to the west, Detroit is to the south, and Selfridge AFB is to the southeast. Cleveland Center has a non-radar sector to the east of Flint as well out there in booneyland.
The boundaries aren't always intersections like POLAR (and in fact I don't actually know if POLAR is the actual boundary or not; I just know the frequency change happens around there). That said I can't think of any off hand that would more clearly define the boundaries.
If you're in the Flint area, you can always schedule a tour there, too.