Dogface - Not sure where Brad found his but I got mine on ebay. Some are old bus, police or ambulance lights, and others are intended for general automobile use. I've found the general use ones tend to go pretty high as far as cost so I hunted around for a bit and snagged this one for under $100
It's a c.1921 Protex Signal -
http://farm8.staticflickr.com/7180/6924279335_7be8101d08_z.jpg 
