Most HA glass for the Beselers and Omega is much thicker. I also believe it's tempered and has to be cut to size before it is tempered. I tried KBH with no luck. One place I've found might be http://www.newportglass.com/ but I haven't contacted them yet and know nothing about them but it looks like they might do small orders.
I ended up finding a new head and decided to use it. I had to remove the lamp assembly to change the spacer from one to the other so it should not be too difficult to replace the glass. The filter assembly is held together with a couple bolts.