I got this on Amazon and it is awesome! It's really loud, so it's not a polite little horn. It's a- get the hell out of the way-horn.
My Mom rides a lot at night and is always worried about hitting deer or them hitting her. And due to watching way too many CSI shows, she worries about being abducted or whatever else happens on those shows. So this is what we got her for her birthday this year.
Click on the image to play the video.