The best example of an original county courthouse in Grand Rapid's "metro area" is in Ionia, Ionia County:
Tyler Card's flickr album
Built in 1885 as a Classical Revival structure, it is the largest structure ever built with Ionia sandstone.
Sadly, only three out of seven counties in Metro Grand Rapids still have their original courthouses (three out of nine if you include Muskegon and Mecosta Counties).