My understanding is that there aren't too many non-German p99's out there. My inquries to Walther-America (w/in the last month) concluded with the rep (kfredette) telling me that "The P99's are manufactured in Germany."
I'd love to squash this apparent myth that there are P99s that are currently manufactured in the US. There used to be some 40 caliber P99s with U.S slides, but that was several years ago and only numbered 3000 units. From what I've been told, all 9mm's are German made.
Having said this, I'd defer to someone else with more expertise on the subject than me (Mr. Costanza? Dr. Lunde?).