If you can find an old electronic police or fire siren -- they almost always come with a "Horn" feature (watch the show COPS or listen the next time a similar vehicle passes and you'll hear what I mean).
I had one wired into the horn button when I ran volunteer Fire/EMS. Basically it consisted of a control unit (including a knob for selecting different siren tones), wiring (you could setup an easy push-button, or wire up a horn switch relay if the unit is equipped), and a speaker (I used a 100-Watt variety with a large bell housing). It's been a while, but I bet the old stuff can go pretty cheap on eBay. Of course, you'd have to disable the sirens or expect some trouble.
As far as effectiveness, it was loud and different enough to get the attention you needed. Not sure about the laws of its use in CA.
I used an old aluminum fire extinguisher for the air tank and a heavy duty truck tire pump from harbor freight for $39 on sale. I used a few fittings and hoses from the hardware store to tie it all together with a 150psi pressure regulator from the same hardware store.
I have less than $150 total in it. I used a smaller sized fire extinguisher tank that fit in the front bumper and is very quick to fill up from empty so the compressor doesn't stay on for long. It is a pretty nice setup and nobody can ever say they don't hear my horn anymore
My stock horns took a crap on me, couldn't make a peep. So I got a couple of horns from JC Whitney (jcwhitney.com). They have a good selection. I settled on one 130db hi tone and one 130db low tone, part #'s MR710714 and MR710716 for $14 each. The two different tones compliment each other well, and it's much louder than stock. They also fit like stock and are easy to wire in. They also list a 140db horn, train horn, ocean liner horn, air horns, and novelty horns!