I just pulled a whole assembly out of a boneyard for $10. Replacing the whole assembly with a used one might be cheaper than buying a new piece of glass. And you'll have a spare in case a city bus decides to get a little too intimate with your brick.

To take it off, you need to take the door handle and panel off (or at least to top corner of it), take the rubber tip on the mirror's handle off, and carefully remove the black plastic triangular trim plate (on the inside of the door, where the mirror's handle is). With the trim plate off you will see a metal gizmo with three wires coming into it from the mirror that determine the direction it turns. Don't be tempted to take the crimped wires out of that harness, as I was, or you will ruin it and have to find another car with a good mirror assembly, as I did. Anyway, there is a metal washer thingy at the end of the metal harness that you have to grab with some pliers and pull out, and once you do that the whole thing comes apart and then you can take it off the door without a problem. You can do the whole thing with a screwdriver and some pliers, plus a long phillips screwdriver to take the door handle off if it hasn't already been pulled off.