The Siddur used by most German communities nowadays is the Sfas Emes Siddur published by Victor Goldschmidt Publishing. However, even at its first publication in 1799, the nusach had been changed to reflect some of the customs that were being practiced in Eastern Europe (Poland). You can get a copy of the Sfas Emes for yourself from Ephraim Rosenstock in Washington Heights.
Rabbi Hamburger in B'nei Brak had published a more authoritative one, but it's out of print. Rabbi Hoffmeister of Vienna also published one (I have a copy and it's great), but I don't know if it's still available.
This one is nice too, but it's currently only a work in progress. (Note I am working on it.)