Well...nowhere online. I've got a MicroCenter nearby. They claim to have a bunch in stock in their store, and I've heard other people claim that they bought theirs there. I've got to imagine that there are other electronics shops in a similar situation.

they're ramping up the availability at the moment, adafruit has them I think http://whereismypizero.com/ - some places only have them as part of a kit admittedly, but I managed to buy one on its own. I'm confident availability is getting easier, and the new version has a camera slot too.