(One immobilien told me that the landlord of a house won't rent to Americans. Are they allowed to discriminate like that?)
Sure. If you had a house to rent out, you'd expect to be able to chose who you rented rent it too, surely? I know I would! Although you / I might be smarter than to make your aversions so overt!
Most of the local papers consoliate their ads in (something like) clickrheinmain.de (I've listed it here before but forgotten it). The one-off weekly publishing of ads still exists but is slightly old-fashioned in today's virtual world, in that you do not need to rely on it. The main local paper is something like the Wiesbaden Courier but I am sure this info is easy to google.