Craigslist
I'm in San Diego and I troll both SD and LA's Craigslist. In Moreno Valley a guy is selling a Bike Friday Silk (8 speed IGH with a Carbon belt drive and 20" wheels) for $800. I mulled it over, but I want the 11 speed version or the Tikit. I just missed a Brompton for $700 (I just couldn't shake free from work).
Deals can be had. You just have to check everyday. In almost 3 years of trolling, I have only seen one Birdy and one Mezzo. I have seen many Bike Fridays and Bromptons.
As a point of reference, I own a Downtube Mini, Dahon Mariner, and the Crius 14.