For making .400 Cor-Bon brass, I like to use Winchester and Federal .45 brass, since they're the thickest of all the commercial brass. They last longer than the thinner offerings from Remington and Speer. There's no need for +P brass.
It works best to use new brass, as you'll get fewer damaged cases when you neck them down. You can use once fired brass, but work hardened brass doesn't do well when trying to make the conversion. Another thing to watch for is round case mouths. If the case mouth isn't round, it will wrinkle, and ruin the case.
Hope this helps.
Fred