Go to Ann Arbor. Go to Encore, it's amazing. It may close this year. "No" new records!
If you get there early enough, you can hit PJ's, it's on the second floor of the tiniest little triangular block, it's easy to miss. If you are willing to move a box or two out of the way, he has a great collection of imported rock.
Also, Wazoo, upstairs, also sell CD's.
Don't know Detroit, but if you have a car, these are worthwhile!
Paul