I did it a cheap but fully functional my whole set up is ran by a 12v harbor freight compressor. At first I tried the cheapest one they had but I wanted more power and the pressure gauge was weee bit off more like a whole 100 psi. So I decided to go with the next step up it was only $17 why not this thing is great small quite and some what powerful the only draw back is the max is 100 psi I have pushed it to 135 but it does it like it. My pressure switch is set at 135 off which is just to high for me. If anyone has a idea on tampering with pressure switches hit me up the main spring adjustment has been welded for “safety”. But anyway my air tank is very small you might laugh it is a 9 oz co2 tank for paintball. But that is plenty for my horns they are very low pressure horns they work at 30 psi sometimes lower. So the is no need for me to have a $200 compressor.
P.S. I went to a junkyard ask for some air horns and the turned me down he said the did not have any he and I know he did maybe I am just to young in his eyes I am 17 and he was like 60isss
Sounds like you have put together a good combo for a cheap route for a quick burst for your horns, might be just what some folks on a budget are looking for. You need to get some video up of your setup in action!
Here is a video of the set up
http://youtu.be/Ntu0UTNV8bc
Nice, you could get a couple of quick full honks or a burst like you did in the video…enough to get noticed. 
If you don’t mind sharing with the worldwideweb, you should post an itemized listing of your total spendings for your setup…for those looking for exactly what you have, on a budget.
compressor $17 (harbor freight)
hose $15 (harbor freight)
9 oz co2 tank $10 (walmart if the have paintball supplies)
pressure switch $15 (tractor supply has some)
solenoid $10 (amazon)
horns $25 (I got mine from a auction but junk yards should have some)
brass fittings $5-$20 (harbor freight has some cheap ones)
the co2 tank you have to do some modding for it to work I have found you can make a adapter by taking out the valve and cutting it then you thread the side with only threads in to the tank(the tank is 3/8 mip not npt it has the same thread count just not tapered like npt you maybe able to find one). Now you need some thing that is 3/8 npt to thread on to that and bam instant air tank for cheap and the tanks can handle high pressure they are rated for 1800 psi.
The price of this set up is all depend on how much you have and know to start with. I have a 1 gal tank I could use that would be plenty of air.
Nice, that’s good stuff man.
Depends on one’s definition of “plenty”. 