I haven't bought any 7.62x39 yet, I will be as soon as I pay off the WASR and SKS. It's good to know that my source has a full pallet of it. ($2.50/box, East German 1970s production)
As far as other calibers, my bricks of 22 come from Wally World, 22 mag from Collector's firearms in Pelham NH (T-Bguns on gunbroker, but I dont think they're moving ammo through there), all surplus calibers so far (7.62x54R, 7.62x51, .303, etc) also come from Collectors.