You seek out stained glass windows at antique stores, flea markets, etc., looking for the right size. OR you go to a glass vendor that can do custom work and you have this made. Here in Raleigh, NC, I'd go to CG&D. Don't know where you are, but look up custom glass in the yellow pages.

If you Google stained or leaded glass and your city, it should bring up vendors such as our studio.

These were custom fabricated for my clients at 4th Street Woodworking in Emeryville, CA. They used to be in Berkeley, CA.