How to get a German Keyboard in the US?

This is probably your best option, a lot of people use KB Covers for this purpose, since it's cheaper and easier than buying and installing a whole new keyboard, plus you can always remove it if necessary or if you want to sell it, etc. Whenever people are looking for AZERTY keyboard I refer them to one of these, but here is their QWERTZ model.

You don't have to do all that if you already have an ISO European keyboard layout. I have a Spanish keyboard Macbook Pro that I use to type in German. All you have to do is go to system preferences> language & text. Click on the Input Sources tab and select the languages you want the keyboard layout to operate under. It will now display the flag of whichever country your language originates from on your menu bar.

You can now cycle through how ever many keyboard layouts you desire at a click. Depending on my needs, I use a Spanish-ISO, German, and U.S. keyboard. If you already know how to type on your layout of choice, there is no need to look at the keys. However, if you don't know the configuration of the keys. You can access it on the Show Keyboard Viewer which is located on the same menu bar which contains the keyboard selections. That should help you get acquainted with the layout.

If you still need further help or can't type without using the hunt and peck method to identify the keys. Then purchase one of those thin plastic keyboard covers which contains your desired layout. They're plenty of them online for sale. This only applies if you already have or plan to purchase a Mac with a European keyboard layout. I don't know what to tell you if you only have a U.S. specific keyboard.