You can buy them for 40 or 80 bucks on-line or you can pick up a piece of plexiglass. Use your old one as a template. Cut and drill your own and get the exact same thing. Watch for details like bevelled screw holes. (Mine are bevelled only on one side.)
I had my wind wings and sun visors made from Lexan in 1997 and have never had a problem with them. It cost me $25 dollars to have them cut and all the bevel work around the edges done. Probably cost more now, but the glass place had a lot of Lexan pieces laying around that they had left over from doing some office windows.