We got our pallet of them from OBK as you know. If you're near the border it might make more sense to do a road trip south to get them though, it worked out to $40 per keg that way but they were very used.
So the best deal I have found online other that waiting for some to pop up on Craigslist are in Canada!
Ontario beer kegs (OBK) 4 for $198CA ($50) shipping. I did add a pile of other items since I was paying for shipping.
Not sue what the cost to ship to the US would be but with the dollar exchange you guys in the US might want to have a look at these if you need some. They came pressure sealed and in not to bad condition.