you could try here:
PORSCHE BOXSTER SPEC - PRECISION CHASSIS**** WORKS
or, you can get a set of cat-less pipes from Cargraphic (around 250 GBP)
http://www.cargraphic-ms.de/stepone/data/downloads/d6/01/00/986_Pipes_10_05.pdf
(page 4)
and add one of those cheap chinese mufflers from e-bay