Take a look at this 417 model:

this is what I am going with just got to find the time to go pick it up, since they are local.

Here are the specifications:

AIR POWERED - produces a powerful "BIG RIG TRUCK" Sound. Two (2) durable red plastic trumpets. Easily installs in any 12 volt vehicle. The compressor is heavy-duty and is maintenance free. If you want a air horn for your vehicle that will produce a "BIG RIG TRUCK" sound, the Model 417 is the for you. Comes complete with all mounting hardware. Instruction in English / Spanish.

Trumpet Size:
Short Trumpet - 11 5/8"
Long Trumpet - 12 1/8"

Only cost $39.99
Here is the site with sample sounds:
http://www.wolo-mfg.com/air.htm