We use an automotive horn available from any auto parts store. However the siren's are a great idea, as they better catch the attention of spectators who may be walking on the course.
If you are looking for pro level sirens and rear lighting, Kartek is the place to go, as most desert racers go to them. Sirens, horns, blue rear lights, amber rear lights. Wiring and buttons can be found at any auto parts store, or you can use high quality switch panels designed for racing.