I added a set of horns to my work truck. Here is what looks like a good start; Triple Train Air Horn Kit + 110 PSI Air System 150dB+ - eBay (item 360319685244 end time Dec-15-10 04:19:26 PST)
My installation

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I have replaced the pump in this system with another from Sams club for $30.00
I can build a couple of systems for the $500.00 they want for the one.