They pop up on eBay from time to time, but you can also try the eLightbars board. They are located at
http://www.elightbars.org/ . That's also a good place to find many older lights and sirens, as many of the members here who have had success there will tell you (I've been a member there for years....good stuff).
I'm still waiting to see some of those in-service pics.