You can also use an automotive horn. I replaced mine with a Hi/Low Tone dual set from Wolo. They're designed for cars, round, and easy to mount. They also put out a decent amount of sound without using a direct-wire technique with a relay -- they don't draw enough amps to justify the direct-connect, so they won't overload my horn circuit/switch. I got mine from a local Mills Fleet Farm store, in the automotive section -- same on-line price, minus the S/H. Cheers!
1979 CB750K (sold, 2012, but not forgotten)
1983 Kawasaki 440 LTD Belt Drive (sold, 2011)
1993 Kawasaki Voyager XII