Of course you can easily get a cheap prepaid card in Germany but I doubt whether you will end up with a cheap fare because it may get very expensive if you do not have a flatrate and I don't think it would be good to buy a flatrate just for your stay. There are Telekom Hotspots (W-LAN) at many places available such as train stations, airports, ICE trains, Mc Donalds restaurants and more. Maybe that would be an option for you.