I have an apple lap-top with an external keyboard. My computer has the capacity to compose in German (a capacity I need) but my keyboard is in English and many crucial keystrokes (extra characters, punctuation, etc.) are different on the German layout. Since I have to plug in my external keyboard any way, why could I not buy a German external keyboard to plug in when I have a lot of German stuff to type? Where could I buy such a keyboard?
You don't need a German keyboard to use the German keyboard layout. You can enable the German keyboard layout from System Preferences > Language & Text > Input Sources, and you can use the virtual keyboard viewer to look up the positions of characters.
Even if you buy a keyboard with a German keyboard layout, the input source won't change automatically when you connect the keyboard or press some keys on it.
You can buy keyboards for different keyboard layouts from store.apple.com.
Note that there are some differences between the Apple and standard PC versions of the German keyboard layout. Particularly the five ASCII special characters under option are all under different keys. You can download a keylayout file for the PC version from http://www.nullpointer.de/stuff/keylayout/.
ISO (European) keyboards have one more key than ANSI (U.S.) keyboards. The extra key is a dead key for
^ in both the Apple and standard PC variants of the German keyboard layout. In the version of the German keyboard layout that is included in OS X, option-shift-6 enters the same dead key state as the extra key.