We recently made a trip through Nashville and the guys at Rebel Brewer said they're sources were drying up. Pretty soon they'd be selling only new kegs.
I just checked they're site, almost fifty bucks for a used one. It seems that either a lot of people are getting into homebrewing and the whole supply/demand thing is catching up with us(basically they can up the price, cuz they'll know we'll pay for it), or all the used ones are slowly leaving us...