Some nice ones available on Ebay. Link below is to one that I bought. IT is very nice and pretty cheap. If the link does not work, just search ebay motors for "porsche license". Lots of them available.
http://cgi.ebay.com/ebaymotors/PORSCHE-black-license-plate-frame-SCREW-CAPS_W0QQitemZ8010803916QQcategoryZ50451QQrdZ1QQcm dZViewItem