I purchased mine from Siege Engineering's E-bay store for $185. They are not showing any product on E-bay or their website. There are many other systems on E-bay at; air horn, eBay Motors, Electronics items on eBay.com. One thing is the system using a storage tank allows you to be able to air up tires. My gallery at; 2001 Ford Econoline 4x2 - Tech Photos has pics of the system installed on my tow truck. Sets run from @$200 ↑. I recommend a system that has the tank and compressor as separate components. I wired mine so that a relay is open if the switch is in the off position. The relay closes when the stitch is on. This allows power to pass to the air horn valve solenoid when the regular horn receives power from the horn button. Aux switch off = regular horn only. Switch on = regular horn + air horn.