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.

I've thought that too  but thinking something doesn't mean its a fact, but Id been making observations on quantities of standard materials  that were accumulated from various sources.

I was wanting cotton and it seemed I was getting less of that compared to the rest, even after switching to  yielding food, after checking where specifically I might find cotton, but got loads of elixir instead, which is also  found in farm tiles. 

Things appeared to change when I stopped being desperate for cotton .  I gave up being desperate, I built something else instead, that didn't need the cotton, only now do  I have the cotton that I  had wanted , and am not sure I am going to use it now.

Similarly, In another game I was playing I was hoping to get boosts, but was getting the other items, when the need became less urgent, the boosts seemed to be more frequent again.

Some of it might just be coincidence, or the fact you are not yielding in things evenly spread out,  for example , iron yields the slowest and there's less in the tile , where  food has the most and yields fastest.  

However  the packs are supposed to be tied in with your individual usage, this being true, its not beyond a stretch of imagination that some similar algorithm is applied to finds. 

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.