When I got my first 458 lott brass was pretty much a do it yourself deal. I found some once fired 416 rem mag brass, necked up and trimmed to length and used 458 win mag dies. Next lott the round had become a factory offering so I could buy brass and dies for it.
You can make 458 win mag brass out of any belted mag case 2.5" or longer, though necking up brass shortens. So 338 and 300 win mag are options, anneal before necking if the brass has been fired. You can also use any of the full length belted cases, neck if needed then trim to length.
