Finding 7.62x39 Boxer primed brass is kinda tough. I'd suggest buying the Fiocchi or Prvi Partizan 7.62x39 ammo in bulk. The brass is good, and the rounds are just a touch above 50c/round.
The cheapest bullets are the ones you cast yourself, if that's an option in your case. Barring that, you might want to look for 100 to 110gr bullets for the .32 S&W/S&W Long/H&R Magnum cartridges. They are typically 0.314" and would probably work great in an AK's action. They most certainly work well in a Mosin, but that's a bolt gun.