I see some seats/bars listed for sale, but they are old listings and I doubt they are active. Please contact me directly if you have one to sell that would install in an '01 Boxster S.
I have a infant baby seat on e-bay right now. I used it a total of 4 times and my daughter is now too big for it. This is the Britax/Porsche seat with the seat belt on the bottom for the air bag de-activation device.