For a good used one - try Woody (ItsNotANova) at 512-925-9171.
If he doesn't have one (they go fast when he does) try ebay seller QualityPorscheParts:
02 Boxster S RWD Porsche 986 R Black ROLL BAR TRIM PASSENGER WIND BLOCK 81,280 | eBay
They're asking $90 but if you offer $70 they'll probably accept your offer.
For a brand new one, Pelican has them for $153.25 plus shipping.
The Porsche part number is 986.561.584.00