a.v.s. sells relays, i used one of these starter solenoids on my dc5000, always worked good for me, all you have to do is put power to one side, the opposite side goes to the compressor, then u run a switch or a pressure switch to one of the small terminals
http://www.napaonline.com/MasterPages/NOLMaster.aspx?PageId=470&LineCode=SME&PartNumber=701847&Description=Starter+Solenoid