I got one of them for my old Sportster and it works OK. I got it at Pep Boys. You can get either low or high tone, and they also have some that are supposed to be a louder version. I got the high tone in the standard horn, and I think it was a FIAMM horn.
It's far from being a Stebel but is better than the stock horn. The louder one might be a better bet. I tried finding them on Pep Boys website but it sucks for searching.