Chuck,
This guy?
If memory serves me well, Erik is right. The only kit I can recall including a reasonable replica is the JoHan Plymouth Police Pursuit kit. And, he's right - it's blue. But the JoHan lenses can be stripped of color in the purple pond.
I don't recall any aftermarket source for this particular beacon, but policecarmodels.com has a similar piece. I think it's a little undersized for the Unity Spitfire, but maybe it will do for you.
They have the most comprehensive inventory of aftermarket emergency vehicle stuff.