There are a few people selling them on Ebay too
a123 20ah | eBay
You can also buy them directly from China
http://www.alibaba.com/showroom/a123-20ah.html
From what I've heard how these came to be is that the factory that is making them had a contract to make them, tooled up to make them then something changed and A123 systems was no longer going to use that factory, no idea who's choice that was, but the factory decided to make them anyway and sell them on their own. so as I understand it, it's a little shady, but they are still good cells and people are using them for good projects.
The same kind of thing happened with the E-tek motors, I did some work for one of the guys who helped design that motor and they tooled up in China to build it but before it was supposed to be released they found out that the factory had opened early and was selling it on their own, law suits fallowed and the improved version of the E-tek motor was never released.