Where can I buy an A/C unit cheap?

[Mark Beekman]

I'm having an HVAC guy out next week to clean out the ducts on my house and to turn on the furnace. I'm eventually going to want to have A/C put in, so I figure why not now? Anyone know of a site where I can order an A/C unit cheap? I'd like to buy it myself and just have the HVAC guy install it. The house is about 1,800 s/f, so I'm not exactly sure what BTU to get.