Last I checked the local Gamestops still have cases for their DS games. There are a LOT of Gamestops around here, though.

There's a few good places to stop for game hunting in the suburbs, though I'm not sure about Detroit proper...

Destiny Games in Canton is probably the best place I've seen around here as far as an independent store front goes. Good selection of stuff from throughout the eras, and decent enough pricing. There's also Gamers Ave. in Garden City, which is attached to Game Yard, a LAN gaming center. They've got a good array of classic games as well. Both places are on Ford road, in the western suburbs.

Downriver, Taylortown on Ecorse road in Taylor is a pretty sweet flea market with three or four booths dedicated to video games - mainly classic ones. You can find some decent deals there as well. I understand that another game shop down here is relocating into Taylor but I'm not sure if they've reopened yet.

If you go into the northern suburbs I know of Flip Side Records in Clawson, which has a section for classic video games. Good prices on some of the stuff, as well.