When I bought brass for the M2HB I got two 55gal. drums from a military auction. It's about 10,000 pieces. All same lot. Make damn sure the lot isn't a shitty one though...
When I got Lapua brass I got it direct from mfg. in 1000rd. cases. Much cheaper that way.
1moaoff pretty much summed it up. Buy in bulk, buy as much as you can, prices never go down, only up. Save and wait for a sale and clean 'em out.