The Porsche parts catalog outfit Performance Products (
www.************************************************************************************************) has a number of them from about $100 to $200 depending on material, protection, etc. Can't particularly recommend one, as the ones I have are cheapo indoor covers just to keep the dust off. The one good fitted one I had on my previous 911 was a heavy denim-like material. Very nice, but it was from Hollywood Sports Cars and I don't think they are even in business anymore.
After I bought my Box I was at one of the local dealers and one of the service techs was applying a clear bra to a Boxster. A conversation ensued and the tech said he personally preferred the lined vinyl removable bra from Porsche. His knock on the clear bra was that they discolor after a while and have to be replaced. How long a "while" is I don't know.
The vinyl bra is a different animal and kind of a pain to put on and take off. The instructions that come with it caution against allowing water to dry under the bra, and a couple of other caveats as well.
If you want a put-it-on-and-forget-it product, the clear bra is probably the way to go.