Not many child seats will fit in the back of a Porsche 911, 964, 993, 996 and 997. Porsche child seats are expensive from the main dealer, there are other child seat for Porsche that will keep the little one happy and allow you your Porsche 911.
I understand that the Klippan Futura child seat fits in a Porsche 911. You may need a seat belt extension. There are plenty of suppliers of car child seats. In the UK you can find a suitable seat at Quickfit, Halfords or one of the larger child seat retailers.