Pentosin CHF 11S
if you don't have a manual, contact the dealer and they can supply you a new one ( or you can probably find one on ebay ). also a lot of users recommend getting the workshop manual. from what i understand it has an answer to every question in the world...
partsgeek sells the fluid for $19. not sure of the price anywhere else. ( not an endorsement or anything for partsgeek, but i do use them from time to time for parts ).
http://www.partsgeek.com/catalog/2001/porsche/boxster/steering/power_steering_fluid.html
download a manual here (pdf link):
http://www.cannell.co.uk/Boxster_Manuals/986%20owners%20manual.pdf
you can also let search be your friend.
Power Steering Fluid - question