Just posting this for the archives, from another thread.
My dual horn ended up coming from Carquest Auto Parts of Cadillac, MI. They shipped it UPS. Phone 231-775-9772. Their part #SGT 72223. Price $38.43 (in July 2009).
If the part is still available, I would think any Carquest ought to be able to order it (but maybe not).
I would say the dual is a definite improvement - but it still just sounds like a better car horn.

Without a place to mount real horns, I'm not sure there's a better option. The thought crossed my mind of getting two of the dual sets, and mounting one of them on the starboard side so at least all the sound wouldn't be coming from one side of the boat, but then I laid down until it went away.