All invaders and tasks can drop steel or any other "normal" material.  Also iron mines can drop steel/diamond/lead and gold & silver can drop steel or any other normal material.  I think the best way to get steel is to farm lv. 5/6 iron mines.

P.S. I am firm believer that whatever material you are most desperate for at that moment will be the one material that Plarium won't give you.  I have no proof of this, so maybe I am just unlucky:)  

Thanks guys. Usually when I can't get something because those invaders are not out, I go hit ubers. But ubers take the most energy from you. They are super-tough! I've got some poster board. I am going to put a few sheets of it on my wall and write everything down so I can see it all in one glance rather than toggle through a bunch of pages online.

Thanks TMC.