When you're cycling, it's important to have some kind of sounding device. A bell can be heard by pedestrians and cyclists but to be heard by drivers, you need something louder such as a horn. Car horns can be heard by drivers when their windows are rolled up. In this setup, the horns were attached underneath a bike basket and the battery and relay were stored in the basket.
The parts can be found at hardware stores, electronic component stores, and battery stores. Canadian Tire and Lee's Electronics carry the materials.