Don't buy them retail or you'll get your head ripped off at your ankles.
I found the cheapest source for Bosch sensors ($113.00)that have the correct plug on them here:
http://www.oxygensensors.com
Here's the direct link for the 2.5 motor's sensors that I bought:
http://www.automedicsupply.com/catalog.php?&engine=H6-2.5L
FWIW: Buy and replace all four at once unless you want a lot of future CE lights and really bad performance from your Boxster.