try the berlin freecycle group!
Location: Places >> Europe >> Germany >> Berlin >> Buy and Sell
Single mattress for free
I got this mattress which I don't need anymore. The looks of it are not great, but it's still nice and fluffy and fully functional. I live in Schöneberg, close to Nollendorfplatz. Let me know if you need it.
Here's some pics: http://postimg.org/gallery/2cqpemx8/3204ad4a/