I bought them from MINImania.com - I'm not sure of the brand. The install went just fine. I think I might have been their first airhorn on a MINI install, but they did a great job.
The horn has stopped working, but it's because of horn, not the work. I just bought a new one, it's a FIAMM. It's two decibels louder than what I had myself. I plan to do the replacement myself, since the wiring is all in place.