Hi garlandw,
Looking through the Pelican Parts catalog i found a low side valve for a 2003 Boxster (
Part#: 996-573-465-00-OEM)
Visually it does appear to be the standard fitting you find on the low side of an A/C line so a normal R134 recharge kit should work for you.
If you find the recharge doesn't last long, then you may have a significant sized leak. In which case you can
inject the system with dye and use a
UV light to diagnose where it is coming from.