Very often I've got questions for addresses of former German soldiers (Navy/Army/Airforce) by people who met them and wanted to see them again. Or you're looking for someone who served in the German army in WWII ...
There are some institutions that might know it. Here are the snail mail addresses (will probably take long time until you'll get a reply!):

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U-Boot-Archiv ( Has al lot of answers about all WWII submarines)
Herr Bredow
Altenbrucher Bahnhofstrasse 57
27478 Cuxhaven
++49 (0)4722-322

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archive

Bundesarchiv
Aussenstelle Aachen-Kornelimuenster
Zentralnachweisstelle
Abteigarten 6
52076 Aachen

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military archive:

Bundesarchiv
Abteilung Militaerarchiv
Wiesentalstr. 10
79115 Freiburg

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military historical:

Militaergeschichtliches Forschungsamt
Zeppelinstr. 127-128
14471 Potsdam

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for lost soldiers during WWII:

Deutsche Dienststelle fuer die Benachrichtigung
der naechsten Angehoerigen von Gefallenen der
ehemaligen deutschen Wehrmacht (WASt)
Eichborndamm 179
13403 Berlin
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military historical museum:

Militaerhistorisches Museum Dresden
Olbrichtplatz 3
01099 Dresden

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for search to people of the bundeswehr look at http://www.reservisten.de

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for historical People (Admirals, Commanders...)
Wehrgeschichtliche Sammlung der Marineschule Mürwik 

Marineschule Mürwik
z.Hd. Fregattenkapitän Dr. Jörg Duppler
Kelmstr. 14
Flensburg
 

It'll help if you write in German. ;-)
It's the only way I know to get addresses!