afaik, you can't buy the glass on it's own from the dealer, but you can
pick it up from a junk yard.  don't try to prize the glass from the
backing plate - tilt the plate until you can get a screwdriver under the
gap and you should be able to figure out where the ball socket is
underneath.  then lever it out gently.

apart from price, the other advantage of practice in a junk yard is that
it doesn't cost you anything to experiment on how to do this if you make
a mistake.