Mark; do a search for Fiamm and you'll find several sources, although I could not find a Fiamm website.
Now if you want something REALLY loud, look for air horns by Buell or Kahlenberg. They are earsplitting, although I don't know if they will fit in a GT40.
Having said that, my Mondial T has Fiamm horns and I've never had a problem getting anyone's attention. Have fun.