As a few post have mentioned.. Quick and easy answer is
http://www.tpcracing.us/
Look on the following site for a few cars that have already had this turbo modification done
http://www.planetporsche.net/cayman-boxster-modifications/
Much bigger bang for buck than swapping to a bigger engine, or trying to put a 911 turbo engine in, which from what I understand is much much harder to do than say a carrera 3.6 or 3.8.
Or you could do what one of their customers did and put in a 3.8 and THEN turbo charge it. Best of both worlds.