I was fortunate to get rifle cases back in about 2004 before ITAR made that a problem. I have about 80 cases now from 2 different batches.
the 24 gauge shotshells reformed to 577-450 are the best option now unless you get someone with an import licence, but then I dont know of any company making new 577-450 brass.
just for interest I have a load using 38grn IMR 4227 and a .458" 405grn bullet avg 1560fps but you need filler, and remember I'm using rifle brass not reformed shotshells. Use Caution