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Shelleytk: There's the Fulton Street Market in Grand Rapids itself.

Also, there was a great article in the New York Times about Michigan fruit markets that mentions one amazing-sounding fruit/vegetable market about 80 miles south-southwest of Grand Rapids:

Fruit and vegetables are king in Benton Harbor, the heart of the legendary Midwestern fruit belt. Since 1860, the city has had a wholesale market that claims to be the world’s biggest for fresh produce.

Once sprawled across downtown Benton Harbor, the market moved in 1970 to a 25-acre site that features three types of commerce. There are brokers, who buy from the biggest farms and supply distributors patronized by supermarket chains. There are also out-of-town wholesalers, who bring produce from elsewhere to sell here, and return with Benton Harbor’s bounty.

I know that's a bit of a hike, but where there's one great market, there are bound to be others, I'd guess. Oh, wait ... the article also says: "By and large, the biggest customers at the cash market are other growers who buy for their own roadside stands or fruit and vegetable markets, which are plentiful in Michigan, Illinois and Ohio."