I had very good luck with Mattman Enterprises
http://www.mattman-enterprises.com/home.htmlVery pleasant people to deal with, prompt delivery, and a good price ($15.25 per plate until 10/31, $20.95 thereafter).
I got the currently correct German plates with a D in the blue field on the left, and the seal and TÜV stickers for both front and rear plates. I believe the plates meet German standards, except that the number I used is CA 230SL. CA is fine for California, but is no longer a valid place code in Germany, and the last part should have 1 or 2 letters first, then up to 4 digits. I call it my poetic license.
Chris Earnest