I read somewhere that someone put a Pilot horn in and it worked, also some (including me) have installed Harbor Freight air horns.
DIY: Harbor Freight air horn install
Just an FYI, if you point them down as I did and others have eventually the red trumpet portion of the horn will fall out of the body of the horn. Mine fell out plus the air pump started failing so I just removed them all together. I think I'd rather go with a better electric horn eventually.