This brass is not going to be available at the local sporting goods store. You are going to have to make your own. You can expect have have 5-10 minutes, on average, per piece of brass, if you make a couple of hundred. If you only make a few, expect close to half hour per each. You need to get some 454 brass, get multiple dies of assorted calibers and start necking the brass down. If you don't go in steps, expect to get buckles in your necks and shoulders. You will have to do some annealing and neck turning also. If you are not ready for a project of this magnitude, let me know so I can have first dibs on that gun.....
Good luck and enjoy that gun. I am obviously envious......
Steve