Found this auction on Ebay...
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=7974454528&category=40016
These suckers are expensive ($180 from the parts counter???) and MUST have the paper tag attached to be programmed to your car.
If your used boxster only came with one key, you should go to this auction, jot down the part number in the picture and call your dealership's parts counter to ask them if it will work on your car.
After you get it, you'll need to take it to your dealership for programming and to have a key blank cut for your car. Bring proof of ownership unless they know you. Key is $25 and cutting it and programming is usually a half hour of $50.
This is cheaper than going straight to the service guy and asking for another key to be made... he'll charge you $250+ bucks and make you come back after he's ordered the key head.