Not to be a complete moron, which I often am, but where do you get the more specialized steel?
You can get pre cut targets from Wideners.com. I think they're featured on their main web page.
I get my pieces free from a brother-in-law who works at a rock crushing plant. They use a lot of AR450 abrasion plate. It's not as tough as AR500, but if it's free, who cares?
And then there's various steel target companies out there.