I just purchased used BMW 740il horns (2) for my 380sl. They are the Fiamm 410khz and Fiamm 510khz. It will sound 100% better than the anemic sound it makes now. Cost...$15. The German car makers use Hella, Fiamm and another manufacture that I can't recall.
BTW, JC Whitney sells quite a selection oif air horns http://www.jcwhitney.com/catsubpic.jhtml?CATID=3126