Yes, Porsche sells infant and toddler seats for the Boxster, but you also have to install the airbag deactivation bar under your passenger seat by bolting it on the seat rail. The bar basically has a seat belt receptacle on it and a wire that attaches to a connector below the seat that after being programmed by the dealer or someone with a PWIS/PST2 computer, will deactivate the airbag once a Porsche baby or booster seat is installed. The Porsche seat has a buckle on the bottom of it that fits into the seat belt receptacle on the deactivation bar.
Here is what the deactivation bar looks like. I have one in my car, and use it with the Porsche baby seat for my 4 month old on occasion.
You can buy the airbag deactivation bar here:
http://e-partssales.com/Merchant2/merchant.mvc?Screen=PROD&Store_Code=P&Product_Code=99680308302&Category_Code=986seat
For the baby or toddler seat, look at the bottom of this page. They are not cheap, but you can usually get them on ebay for much less if you're willing to get a used one:
http://e-partssales.com/Merchant2/merchant.mvc?Screen=CTGY&Store_Code=P&Category_Code=986seat