I ended up buying a lamp with bad wiring for cheap on ebay and stealing the lens and spring from that. 2 half lamps make one whole! I couldn't find anybody making new ones.
The spring is a round piece of metal with bends in it, so it presses the lens outwards against the lugs that hold it in place. I'm not at home so I can't shoot a picture of it. Without it, I would think the lens would be loose and might vibrate around enough to just fall out.