Thanks.
Stebel lowest amps electric horn specs of interest to bicyclists:

TM80-1 500 or 410 Hz, 133dB, 225 g, 2.5 amps, 82 mm (3.15")
TM80-2 500 & 410 Hz, 136dB, 450 g, 5.0 amps, 82 mm X 2

A small 12 V low internal resistance battery such as a lead acid or nicad might be enough; anyone care to check the specs on some?