A friend of mine found an air horn at Canadian tire. As luck would have it, the thing fits perfectly in a water-bottle-holder on his bike, and refills with a high-pressure pump (which it comes with, and worked much better than his hand pump) via a shrader valve on the bottom. When fully pressurized it put out 120 decibels, the pain threashhold of the human ear.

Absolutely nothing in all six universes beats the look on the face of some hothead driver when a guy on a bike honks back.