1/2" thick AR500 steel is what you'll want. The 3/8" plates are usually only rated up to .308 Win. Always better to play it safe and go a little thicker than you might think you'll need.
As for a source, plenty of places to order from online nowadays. Can't help ya on a specific as we can't shoot steel at my local range. I'd look for mount/hanging setups that let the bottom of the target swing freely or has it so the bottom is rearward of the top to ensure the bullets will be deflected down into the dirt and not up into the air. A lot of the 2x4 and 4x4 post mounts I have seen are pretty rigid and/or allow some side-to-side wobble that could let the round ricochet off to the sides or up in the air.
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