I'm almost sure this post won't be helpful...
But wasn't the Walther P99 the combination project with Smith & Wesson and their SW99? Is it possible the magazines are the same and perhaps running your searches based on the Smith & Wesson product other than the Walther project helps a bit?
I have no real idea, but get the sense that the Walther was the more popular product of the two.
Ahhh, I reminisce to a day just 4-5-6 years ago where you could punch up CDNN and be blasted in the face with magazines.