Why a costume manufacturer? Are you gonna break in to their factory and load some of your own?
I would suggest getting your hands on a chronograph and trying to duplicate the velocity of the Doubletap stuff with a handload. A quick search of this site turned up a possible recipe:
200 Grain Accubond Loads for .300 Win. Mag.
It would also help if you could load them to the same depth as the factory stuff since the amount of jump to the lands can affect accuracy quite a bit. Measure one if you have any left, or maybe Doubletap would be kind enough to tell you the specs on their loads.