Bosch and Hella both make a much wider range of Supertone horns than you can generally find advertised on the web. The good ones are significantly more costly and much louder than the little $19 "highway blaster" items from the parts store. I have a few sets of the loud Bosch ones and can order more directly from Germany, if you don't find something locally. As with lights, horns really need a lot of power to work correctly. The stock wiring tends to starve them; run good large-gauge wire (12ga is a good idea) via a good relay. Also, horns have to be mounted properly. Trumpet-style horns need protected mounting so they don't scoop up water and get ruined in a hurry, but disk-type horns (like the Supertones) can be mounted facing front for maximum effect.

The air horns that are offered for cars are silly. They aren't any louder than the stock horns — not even that loud, in many cases — and their higher pitch is less effective.

And in the end, if you really want to pull out all the stops and spend a giant whackload of money on insanely loud horns, go to http://www.hornblasters.com , an outfit who sell actual, real train horns and tanks and compressors to operate them in passenger cars. There are demo videos that are funny for the first minute or so until you realise the people in the vids are being childish jerks.

I have a air tank, with a Caddy air suspension compressor, for use at the track. I also have a Frieghtliner air horn, Hook up a air valve/relay, and an air line to the tank in the bed of my Toyota. Look out idiots, crowding me on interstate on ramps, when I am towing my race car.

http://www.youtube.com/watch?v=tYTz2EbEq8g
please ff to the middle of this nerd video