I ordered these from Amazon. They matched the color of the wheels perfectly and fit snug. They are the newer style of center cap and look exactly the same as the caps on my Cayenne S in terms of crest colors and design. I like the look better than the original caps with the smaller crest and ring around the edge. For $28, I figured it was a cheap gamble but I'm very happy with them. I've had them on the car for a month and they still look perfect. We'll see how they hold up, but like others, I only use soap and water to clean the wheels.
Amazon.com: 4x Porsche Silver Wheel Center Cap Boxster Cayenne 911 993 996 997 HUB Racing: Automotive