I just replaced my stock horn last night with a Hi and Low note pair of Fiamms from Harbor Freight, cost was $9.95 each. Each unit is 132 decibels and the pair is louder than an individual horn. Installation is easy- just attach one unit where the original was and hang the second one with a metal strip from the first one; connect the original wiring to the new one and use two wires about 5 or 6 inches long to connect the two horns together. I've made this upgrade to several bikes in the last few years and didn't need a new wiring harness.
You can search the archives and find additional info plus an air horn that was recently installed.