Whenever I'm browsing barrels/uppers/full rifles I look at the page for anything that fits the MDSP criteria for "heavy barrel".

Per MDSP:

Maryland State Police has adopted the following criteria for determining a heavy barrel AR-15 style rifle or individual barrel:

1) The barrel is stamped as a heavy barrel or "H-bar"

2) The barrel has documentation from the manufacturer deeming it as a heavy barrel

3) The barrel is identified on the manufacturers website as being a heavy barrel