Best Steel plates for Targets?
If you want "just the plates" at a good price (no stands) try Widener's.
http://www.wideners.com/itemview.cfm?dir=1020%7C1023
They sell rifle plates, round plates, or a "variety pack" of several sizes. I bought the 3/8" AR500 variety pack and it has held-up very well to my abuse: 5.56, 30-06, 8mm Mauser, 7.62x54r, etc. anywhere from 60 yards to 150 yards. The harder stuff like Mosin ammo can leave a small pockmark but nothing serious, as long as you don't try to bring it too close.
The plates never break but the mounting bolts will be your weak point, use Grade 5 or better fasteners. I use a wood sawhorse for my stand.
Here's another hint: if you want to use them for both long-rage rifle and short-range pistol use, dedicate some to rifle use and only use them for rifle ranges because once the surface is used for Rifle it's not perfect-enough for short pistol ranges anymore. What I do is paint the "front" vs "back" different colors, i use one side for high-powered rifle and preserve the "back" side for pistol range use. Even with 7.62x54r (Mosin rifle) I've never had one hit hard-enough that it ever deformed the back-side.