Any local steel shop or machine shop should be able to order it.
With regard to what 5/8 AR500 can "take": velocity is the killer of steel, not caliber. M193 shot from a 20" rifle at 25 yards will put very small approx 1mm pit marks in the surface of AR500 steel. If you have multiple hits in the exact same spot over and over eventually it will work harden, crack, and a small "plug" will be pushed through, leaving about a .30-.35 "caliber" hole through the steel. But this is at extreme close range with impact velocities around 3100+ fps.
With a mounting system such as the one I pictured, that has some "give", 5/8" AR500 can withstand virtually anything (other than large caliber A.P.) including .50BMG, .338LM, etc. The lower the impact velocities, the less potential damage to the steel, though I have never seen damage to AR500 incurred with anything other than extremely fast rounds (e.g. M193) at ranges closer than 100 yards.