You can buy this at Radio Shack.
http://www.radioshack.com/product/index.jsp?productId=2062482&cp=&sr=1&origkw=relay&kw=relay&parentPage=search
You want to use a relay which has the lowest coil current spec (the mini relay at the above site looks ideal to me) and a contact current spec greater than 10A. The compressor will draw a maximum of 4.5-5A, the extra amps in the spec is to account for poor construction of parts now-a-days.