Using J. Kozloski's design, I added air horns to my Valkyrie. The horns are $26.00 at your local Pep Boys, Autozone, etc. They are black plastic and come with a compressor, relay, and tubing. In addition, I purchased 12 and 16 gauge wire, an inline 20A fuse, and miscellaneous insulated eletrical connectors. I also used a sheet of aluminum, a pipe bracket, and various spare nuts and bolts. The horns mount under the swing arm and are pretty well hidden. The compressor and relay are under the seat. I'll post some pics in the next few days.
Best of all, the modification as I implemented it requires no permanent modifications to the bike (you can un-do the mod to get back to stock). The only exception is that I enlarged the notch in the left side of the plastic battery cover but cutting it with a knife, and I tapped an existing small hole in the center of the frame cross-member.
You can email me at jdekan@inetport.com