I believe it is a seasonal thing with 30-40 brass.  I've heard of others on another forum talking about making brass from .303, but I believe that is unnecessary.  30-40 brass should become available this winter... and there is usually once-fired available on Gunbroker and EBay.  Another route you could go would be to get some Remington or Winchester factory ammo... and once you've shot it, save the brass and you'll replenish your stock.