Can't help you with Stuttgart, but I asked this same question about Munich last year. I was told:

SecondHandCameraShop, Landwehrstr. 3, 80336 München, Tel. 089-55079595, www.secondhandcamera.de
Cityfoto, Rindermarkt 16, 80331 Munich, Tel. 089-265112, www.cityphoto.de
Photo Bazar, Josephspitalstr. 5, 80331 Munich, Tel. 089-2607313, www.photo-bazar.de
Foto Nürbauer, Zweibrückenstr. 13, 80331 Munich, Tel. 089-296151
Foto Öhlschlegel, Reichenbachstr. 5, 80469 Munich, Tel. 089-2285666, www.foto-oehlschlegel.de

Nothing was cheap! Have fun anyway.

David