well I'm certainly no machinist but I figure that if you have a good drill press, set up nice and square with a brad point bit that you'd be able to drill a pretty straight hole though a boolit. and using calipers you should be able to see if the hole was drilled center.
I assumed that the main intent of the Bushing used on page one was to help ensure that the drill bit was centered inside the cavity. after all it would be quite difficult to peek into the cavity to make sure the bit was exactly where it needed to be.
my idea was just to make your own rather than going to the trouble of driving to the hardware store and trying to find a bushing that either fits or fits close enough that you have to wrap in in foil to make it work.
after all what would fit the cavity better than a boolit cast from it?