You can get at least formable brass from Jamison. It's good brass. It's not listed, but he might make 500-.450, call and see, or get a similar one (500 - .416) and form it to yours. It seems to me he said he was going to make it.
http://www.jamisoninternational.com/Bras...w=10&page=5I bought a Field Patent .450 - 3 1/4" last spring, and it is one of the most fun rifles I've owned in a long time. I load with 120 grains of black and 385 grain lead bullet. Nice, light recoil load about factory spec, and accurate with my rifle at 30 yards.
dave