Questor, thru much experience, try this. Shootall is right on the scrapyard. Most of them will cut a piece for you. As long as your not going to shoot it with a high velocity round,(.223,.243,'06,etc.) mild steel will work out for you. At 100yds., a .44 will have lost enough energy to not dimple the surface. This might happen if you shoot it at say 25yds.
When you have them cut it, have them blow a couple holes in it with their torch. Couple inch's from the corners, and a couple inch's down from the edge. Say about 3/8'' holes. Go to your local hardware store, and get a couple carraige bolts 2'' long, with heads big enough to keep them from going thru the holes. Then get 2 sections of 3/8'' chain, about 1' long each.(make sure they are even numbered links. About 8 or 10 on each side) Then bolt the chain to the back side of the targets. This is do to the fact, if you weld the chain on, and hit the chain just above the weld, you can blow it off. Even with a .44.(I can attest to that from experience!!) If your going to drive in nails and hang the targets, make the chains an odd number.
If you can find it, and it'll cost more, a Hardox400 or AR400 or 500 steel will hold up even for high power rifle at say 100-200yds. Good Luck!! gypsyman