Basically you find in bigger cities some news papers having some advertisment (e.g. in Berlin: "BZ").
Due to the big change I would always use internet. For the middle and southern parts of Germany I have got good experience with www.ladies.de.

This has got a quite good overview of appartments, brothels, fkks. It's off course all in German, but you should understand this somehow, too.

In www.ladies.de you can find as well some flatrate clubs: you pay a flat rate of 100-150€, you can stay there as long as you want to, eat or drink a little and have sex several times (all covered by the flat fee rate). I know that there are not too much of these clubs in Germany, but you should find them.