I know that this is an extremely long shot, but didn't these used to be the same or similar to the lights that were on some aircraft back in the day? If so maybe a call to your local aviation museum might provide some leads on replacement parts.
Aircraft lights are 28 volts. The lens for these is still a stock listed NSN (National Stock Number) current gov price is over $600 dollars. These will probably show up some day in a Gov auction at pennies on the dollar.
The only thing that the Ful Vue lights have in common with the aircraft lights is the lens and the bulbs. Most aircraft are 28 volts and the Ful Vue lights used a 14 volt bulb also made by Grimes Aerospace. I have some NOS red lenses for sale.