So - Norma 458 brass is available and IMHO, better than spending time forming from a smaller bore belted case. It is $2 each.
I got to give credit to jwp475. I did a google search for the OP and came up with nothing. Now he has an option. However....

Hornady brass is also excellent and that is normally priced at $1 each. And that is not some random mix of used brass.

https://www.midwayusa.com/product/1010147153/hornady-reloading-brass-458-winchester-magnum-box-of-50

Before I paid 100% higher price, I would drop a dime and call Hornady. They could be running a batch this week!