I doubt you will find the compressor you are looking for the traditional way...you will need to speak with some heavy equipment companies....Ingersol Rand....etc.
I think it is well worth paying the $60-$80 per year to have it done professionaly...occassionaly you will be able to find a used tow behind compressor that will suit your needs for under $5000.00....but you would never need it again....seems like a no brainer to me...
Post your city/state and I will help you find a good contractor...I have a few contacts around the US....
Sprinkler Solutions, Inc.
Arizona and Colorado
www.sprinklersolutions.net