I have a 40's vintage Airguide Barometer with a broken face glass. I am looking for a replacement glass the glass is 4 3/4 inches in diameter. Any suggestions where I might look for a replacement.

If it were me I'd draw it out to show the specs as precicely as possible and email it to the variety of custom glass companies for pricing.

Here's a few I found searching custom glass:
http://www.customglass.com/
http://www.artglassproductions.com/
http://www.kimble-kontes.com/html/CustomGlass.html
http://www.onedayglass.com/