I noticed that there are many people on InterNations who are searching an apartment or temporary housing possibility.
I work at Homelike where we offer as good as every furnished apartment in Frankfurt. There are a lot people on Homelike who are coming to Frankfurt for a few months (For a new project or a new job).
With some landlords we even have agreements so that our rates are cheaper than the landlord's offer himself! You can give us a buzz on Protected content write me a message. Take a look: Protected content