If you can find a brand you can probably order from the original manufacturer. If no name is available call Thor, they can tell you who made the mirror. If the manufacturer doesn't sell just the glass try a glass shop, they can either measure or copy the glass on the other side. I had one made by a glass shop several years ago, couldn't tell it from the original.
Dave & Myra
One day your life will flash before your eyes,
make sure it's worth watching!