an auto glass shop can cut you a new one at very reasonable price... you need to make it out of glass that is as close to the original thickness as possible AND there the bevelling, if any must be replicated... not a problem for a good shop.... I did it as have many others I'm sure
The best solution to a mirror is have it re-silvered. It is cheap but hard to find a vendor. It takes the guesswork out of fit, bevel and thickness. I HIGHLY recommend Frank Wright Glass | Antique Glass & Crystal Repair
Fast, professional and cheap..All the right buttons and boxes.