A couple of ideas:

Fashion a homemade one from a properly sized wood dowel wrapped with shim stock (fancy name for strips of beer can aluminum) if necessary, in strategic spots, to make it fit the pilot bearing and the disk splines.

or

Scrounge up an old trans input shaft at a junk yard.

Note: I've been successful on more than one occasion just eye-balling it when I tighten down the pressure plate and being able to slide the shaft home by putting the trans in gear and roating the output shaft back and forth slightly while I worked it in.