I'm not sure is this the right place to ask , but do you happen to know if and where it is possible to buy "pikku prinssi" in German?
If the book you're asking for isn't found in the stores, ask the staff if it can be ordered for you. I checked websites of the two big bookstore chains, Suomalainen kirjakauppa and Akateeminen kirjakauppa, and according to their internet stores there are available prints of "Der kleine Prinz". Akateeminen promises to have it in 1-2 days, via Suomalainen kirjakauppa it takes 2 weeks. Contact the stores for more info.