Research is key. First you need to know what you want besides Tip. Like cruise control, heated seats, Bose etc. I nailed that by building a bunch of Boxes on the PCNA website.
Then search comes in. I hit every dealer site on the web within driving distance, as well as using Autotrader and other free services. You'll find some dealers understand the web, and other just use it do generate phone calls. Some BMW dealers the same way, I recently learned.
And don't forget PCA as Grizz said. Altho many of us PCAers seem to overvalue our cars cuz we do tend to take care of them.
And last of all - if you are in the 03-05 range, there are no bad Boxes. Just badly cared for or previously wrecked examples.
Good luck and have fun in you search!
PS - RandallNeighbor is another good source:
http://www.iwantaporsche.net/BoxsterBuyersTips.htm
http://www.iwantaporsche.net/BoxsterInfo.htm
PPS and Mike Focke
http://mike.focke.googlepages.com/mikesporscheboxsterwebpages