Wow. Among the best quality results I got from Google was this post. It looks like that glass was designed as part of the globe's structure. I'm not sure you'd be able to find one suitable for clamping to any globe. It might, however, be relatively easy to build one.
This gizmo, for instance: http://flashlightsunlimited.com/astroarm.htm
It's ugly, but it gives an idea of what might be possible.