Thanks Soeren,
The foam is not related to balsa (the wood) rather it is a Phenolic carving foam designed for architectural models and prototyping. Sign makers also use it. If it is not available locally, you can order it online and have it shipped. The product itself is on the pricey side so I don't know how much it would be with shipping to Germany.
Although I am very impressed with the foam, carving plaster blanks can have just as good results, just make sure to carve it while it is still "green" and not fully cured.
Other materials including heavy card stock, cork, plastic and wood all make great building mediums for architecture.
Hope this helps you out for your dio