For the where, I'm partial to Prairie Nursery and Ion Exchange. American Roots in Ortonville is another great source, but as far as I know they aren't on the web.

As for what to get, well, for Southeast Michigan, there's a fairly comprehensive list here. For other regions, I'd consult with The Wild Ones, The DNR, and The Lady Bird Johnson Wildflower Center. I'd also take a look at the 1800 Vegetation Map for my county (http://web4.msue.msu.edu/mnfi/data/veg1800.cfm).

If you're looking for a book, I like the Revised Edition of the Michigan Gardener's Guide.