I know how it feels. I had a tough time buying a Porsche child seat back in April of this year for a 986. Some dealers told me that they had a backlog and didn't know when they would get them. I even called Porsche to express my concern but since I'm not the first owner of the car, they ignored me. Luckily, one dealer found one in the back buried somewhere.
If it's a 986, you might try ebay. Whenever one does appear, it does go for a premium.
Another option for the 986, is a third party on/off switch:
renntech link
It seems there was a little problem with airbag light, though.
If you have a 987, there's a different approach taken and I believe that it can accomodate non-Porsche child seats. You can view details at the Porsche website under Tequipment.
I learned yesterday that it looks like there will be another approach for 2006:
http://986forum.com/forums/showpost.php?p=28041&postcount=4
I'm disheartened that Porsche doesn't have a higher priority on making a child seat/airbag deactivation switch more readily available.