Check out the AirZounds. It uses an air cannister that you inflate with your tire pump. It is lightweight and loud and effective. I simply love beeping back at the petrochemical addicts who beep at me. You can see their heads whip around to the right to see where that beep came from!
Good suggestions, i think i might order the forklift horn, but I'm worried I'll damage my hearing.
http://www.gcaudio.com/resources/howtos/loudness.html
Even short term exposure can cause permanent damage - Loudest recommended exposure WITH hearing protection 140dB
How about just running high current resistors in series with a regular 12v automotive horn (or better yet... to an air compressor hooked up to some air horns).
Any electromechanical horn would work fine with a ballast resistor to limit the current. The duration of horn usage is very small so resistor losses are negligible.
Oh man, I had the optional four tone-horns on a 1979 Buick Electra. There were two horns in front of each wheel well and man were they loud. I had to be careful because people would literally jump off the ground if they were within a car length in front of me. I started to remove them when I sold the car, but gave up as they were very heavy, large and difficult to get to.
Those suckers would probably numb any part of your body touching the bike.