Stop off at the local Porsche dealer if its nearby. They are always giving away free posters. I just has 2 cayman posters mounted on foam board for my home office.
If you can get the porsche techequipment catalog mailing list, you basically get a 'poster' every 2 months, because the 'catalogs' they send to you unfold into a 2x3 poster.
Ebay has plenty:
http://search.ebay.com/search/search.dll?from=R40&_trksid=m37&satitle=porsche+poster&category0=