I have found the tailpipes, but they are £200 for the pair! You'll need to cut the original chrome finishers off first.
Link:
http://www.type-911.co.uk/exhaustparts.htm Scroll right down to the bottom, and the description is "996 Twin Turbo up to 2002 Stainless Steel Exhaust Tips, twin outlet look, easily replaces the standard ones. 150mm x 100mm with a 8mm rolled edge"
just go to
http://www.type-911.co.uk for further details. They are actually designed to fit a 996, but there is no reason why they can't fit a Boxster.
Would like to buy them myself, but having just paid for a new clutch (sore point!) I'll have to leave them for now!