http://motors.search.ebay.com/_W0QQsassZporscheparts23
This seller has two remote key heads for 98~2004 boxsters that end today. They have the hang tags with the serial numbers required for programming, which are a must! (old keys sold on ebay without the tag are worthless).
If you need an additional key for your boxster, call your dealership and get a part number NOW (just to insure the seller is telling the truth about it working on 98 and up boxsters) and bid on one of these. Dealer charges anywhere from $145 up for just this part alone, so any price south of this is probably a good deal.