Horn relays are everywhere. Practically every car or truck has one.
These should get you started:
http://www.rattlebars.com/mtz/hornrelay.html
http://www.webbikeworld.com/t2/horn-rel ... -relay.htm
http://www.webbikeworld.com/motorcycle-horns/
http://www.easternbeaver.com/Home/Main/ ... s_kit.html
The ones made specifically for motorcycles are smaller and look like they are more water resistant but you don't have to use one of these. An automotive one will work.
Just make sure the one you use is rated for the amount of current you are going to have on the controlled circuit.
Horn relays are a lso good for control other things besides horns.