...brass is(was, I haven't checked lately) hard to come by. Bought my rifle, brass, factory ammo and bullets in a package deal with a fellow on Gunbroker and was VERY pleased. I have about 300 pcs of new and once fired and about 200 factory rounds.
I think Starline makes the brass...check out Gunbroker, they always have some on there even when they are taking back orders.
There used to be a REALLY good dedicated 458 Socom forum but it is down right now, hacked or something...but check out Teppo Jitsu's website for lots of good reading. He is the original designer of the round.
Be sure to watch the short video clips of the 458 Socom in full auto...one is with a suppressor, it is cool stuff!