Ebay is a decent source if you live somewhere without an active enough Craigslist like I do. You're unlikely to get one below $400 there due to shipping though.

On the other hand, I've used one for 4 years now, and I highly recommend it. It really is that damn good, and if there are two things in life you shouldn't cheap out on, it's a bed and your primary work chair, in that order.