Steel 'biners are used for rescue, where there may be a lot more weight than your average climber weighs (150#), including rescurer and 1+ rescuee's, metal litter, etc.
Also, visit your local hardware store, Home Depot, or Lowe's and check the fasteners section... next to the chains and paracord there should be tons of "industrial" carabiners. Nice and heavy all steel and very cheap (about $2 and no shipping!).
