I think that at about 8 years, you'll soon have the other sensors going too, so I'd recommend replacing the whole set.
A friend bought a set from Titantpms.com for his 2007 Carrera around 2014 and they're still working fine. He found a tire shop that simply let the air out of his tires, broke the bead, reached in and replaced the whole unit. It saved him from having to get the wheels rebalanced.
If you go to their website, you can click on the sensors they have listed under Porsche and it will tell you for which year Boxster (and other models) they fit.
It was considerably less expensive than what Porsche was going to charge him. I don't recall the exact figures, but Porsche (Canadian) price was about $200 / wheel and he bought the set and had them installed for a total of less than $200 on all 4 wheels.