Search in the local newspapers for ads. In Karlsruhe, accommodation can be found through:

Badische Neueste Nachrichten (BNN)
Lammstraße 1-5, Postfach 311168, 76147 Karlsruhe
Phone: 0721 789-0
(Housing market on Saturdays,especially on Wednesdays)
www.bnn.de

Karlsruher Anzeiger
Stephanienstraße 25, 76133 Karlsruhe
Phone: 0721 1800-18
www.karlsruher-anzeiger.de

SperrMüll
Free online platform for private ads
www.sperrmuell.de

Wochenblatt Karlsruhe
Am Sandfeld 15, 76149 Karlsruhe
Phone: 0721 97848-0
(on Wednesdays, it's distributed free of charge to all households)
www.wobla.de

Der Kurier (BNN)
Linkenheimer Landstr. 133, 76149 Karlsruhe
Phone: 0721 789-0
www.derkurier-swd.de