For the touchscreen try these guys:

http://www.elotouch.com/Products/default.asp

However a real tablet is the way to go. head over to the OSX86 project forums and search there for the exact model of PC laptop the digitizer mentioned above came from. If it works for them it may well work for us as well. The PC tablets have 256 levels of pressure. The tablet itself is serial and will need to be hand wired to a usb adapter of some sort. The driver for the tablet is called tabletmagic. I am trying this myself as well with a 14" ibook and the screen from a gateway M275 tablet. The screen should arrive on monday. Between the group of us putting our heads together we just might pull this off.