A good air horn is indeed loud. I bought for about a Stebel Nautilus for $40, relay and wiring kit included. It is compact, yet I have not found a place to mount it that I'm happy with, if you come up with any good ideas pass them on.
I got the Rivco dual air horn set. Wired it up and mounted in about an hour and it is LOUD!!!!! My friend and his kids heard me from about 2 blocks away as we were in a group ride last year. Great quality and has been on 3 years no problems