I gave up early on trying to hunt down cheap buck and slug ammo. Like CreamySmooth I bought a Lee Load-All 2 and load my own. A $40 00 buck mould, $30 1oz slug mould, $50 press, and free lead smelted from WW's (though that's getting harder to find.)
I can make 8 pellet (~1oz) buckshot loads and slug loads for $.40 each if buying lead for $2/lbs or $.30 each if getting free lead. Absolute cheapest I have found factory slugs is $.75 each and buckshot $.80 each. Plus I can load up or down in power as I see fit. I have loaded 1oz slugs down to 900fps for plinking and up to 1650fps for dragon slaying (factory "1600fps" slugs only chrony 1450fps when I checked.) It's easy to find recipes for 8, 9, and 12 pellet 00 buckshot loads in 2-3/4" hulls.