Amazon
Get the part numbers from Mike Focke's Boxster pages and then get from Amazon.
https://sites.google.com/site/mikefocke2/oxygensensors
You have a 2001 so your numbers are:
15182 Oxygen Sensor - OE Type Before Catalyst (Front)
15183 Oxygen Sensor - OE Type After Catalyst (Rear)
If you enter these numbers into Amazon search it will take you right to them. I myself has installed the 15183 just a few weeks ago and it is working like a champ (into a 2000 S)
Steve
Edit: it looks like there is now a generic "exact fit" style aftermarket sensor on there that doesn't require splicing for around $30... Most folks have not had luck with the splicing type aftermarket sensors but this one is no splice.... if it was me I would probably still get the Bosch but Amazon has a really great return policy if you wanted to try those others....