They're surprisingly economical from the Porsche dealer, especially compared to what seems to be the going rate for them on eBay. However, if they are not available they will be queued up for a while until HQ in Germany decides to print another batchof them. So if you want a printed hard copy manual, and are not in a hurry, I'd recommend buying new at the dealer.
There is also an electronic copy of a 2003 boxster owners manual floating around somewhere on the internet (don't have a link handy right now).