I don't remember which horn I had (high or low tone), but it was made by Fiamm and is sold in blister packs with Lord knows who all's name on it. (Horns still have Fiamm logo on them though.) I've seen 'em at WalMart, CSK, NAPA, etc.
One thing though, it was certainly louder than the stocker... but it doesn't like rain! Within a year it was toast. Even though I live in rain country, I try to keep riding in the rain to a minimum, so it's not like the horn saw a lot of wet conditions.