onboard air compressor?

[GreenMystChevy]

I am building up my 4x4 and would like to put air on it for a few reasons. I want to install an air horn and I would also like to be able to air up whenever and use an impact gun on the road. The problem is that I don't want to spend the cash that 4x4 places want for these shiny new things.

Does anyone know what or where I can get something that I can wire up under the bed to fill a 5 or 10 gallon air tank that will do the trick? I am not looking for something that outperforms all others, just something that will do the job.