Original caps are expensive even on Ebay and be careful with the knockoffs. Just buy the crests. One of my crests fell off so I bought 4 off these ($38 total) and will keep the 3 for spares. They have double-side tape on the back. Good quality and perfect fit.
Porsche Rim Wheel Center Hub Cap Emblem Badge 987 996 997 911 Boxster Cayman S | eBay
Don't know how particular you are but my caps have a ring around the crest. Some caps on Ebay don't have this ring.