Harbor Freight has a 12-volt dual horn that produces 135 decibels for $14.99.  

135 dB is 'louder than a gunshot' but there is no 'nausea' until 141 dB and you have to get to 144 dB before vision is blurred due to eyeball vibration.

A 152 dB three horn air horn (152dB Vibration is painful and felt in joints; 153dB Throat vibrating so hard it is impossible to swallow) would cost $138.95.