There are a couple of sites for steel that are really pretty cheap for what you get and offer free shipping. www.magnumtarget.com
is one of them. All of my steel is home done. I have some A36 3/8" steel that I had plasma cut into a 2/3rd scale steel challenge size. I'll mount them on posts in buckets with concrete for weight. Then I can move them around and set up all 8 stages.
For shooting steel with pistol calibers you can get away 3/8". Maybe even 1/4". The trick is to hang them so that they are not rigid and the targets can swing to absorb some of the impact. The other thing is you're shooting steel from inside of 10 yards you'll want to either have it swinging or angled toward the ground slightly. Shepard hooks work pretty good for hanging steel and allows the steel to swing some.
Here's a pic of one of my steel setups I made myself.