I would have to say some kind of a spring valve. In a semi to use the horn you just pull on a cord and it will sound the horn, let go and it springs back. It is not electrically activated because the farther you pull the cord, the louder it will get. It would work good to get the valve from a semi, even if you don't mount it on the ceiling of your truck. A friend of mine has "train horns" on his semi. He just tapped into the air tank and ran a line into the cab into a ball valve (like the ones used for natural gas shut-off valves in your house) and then to the horns. The more the valve opens the louder it will be. It works good like this because you can start quiet and then get loud, just like trains. The regular air horn valve in the truck works this way also, but without as much control. Trucks operate on around 125 to 140 psi, I don't think that the horn is regulated, so it should operate at the same pressure. While at the wrecking yard, check out the type of air hose used along the frame rails for the suspension and brakes(on the frame, not from the frame to the brake chambers). I think that it will work good for plumbing an OBA system. It's easy to work with, just cut the hose and stick it into the fitting. It works kind of like chinese handcuffs. To remove the hose just pull back on a small collar and remove the hose. Semi's seldomly have leaks on these lines. In some magazines I have seen electric air horn kits. These are for if you don't have an air supply, which most vehicles don't.
