I want to buy a book from the US-store, but don't have a billing adress there... The book is not available else where. What can I do, i really want to buy that book... Help me please...

You can't use the US store unless you are physically within the US - being in a country is part of the store's terms (terms that you agreed to).

You could try contacting the publisher and ask them to make it available in your country's store.

Or is it available on other ebook apps/stores that you have access to ?

You say "not available elsewhere"? Have you really checked all the other stores like Amazon, Nook, Kobo, Google, Sony?