I don't think any horn will work off the white wire but I might be wrong the lights work it pretty hard. I think one of the handheld air horns would be the way to go and you got me to thinking I could seal off part of my frame and make a small air bottle out of it to power an air horn but then again I'm lazy and will just yell and cuss out how ever gets in my way.
good luck my friend on your horn quest. You know the handlebars would be a good choice for the air bottle My Harley's handlebars serve as an air reservoir for the front shocks. So there I go.