Originally Posted by Superman Prime
The "hard to come by" idea is nice, but think about it. Lex Luthor can acquire "hard to come by" because of his influence and riches. It makes little difference in the end.
And there are a trillion other things you can use for an Achilles' Heel. Personality traits are the best.
I get where you're coming from, there are many ways to nullify Superman in this vein......the most often being danger to others if Superman interferes....and they create the opportunity for Superman to out-wit the threat vs out-punch.
However there needs to be a real mortal threat, and Green K is that threat, and if rare enough on Earth, even the resources of Lex luthor become moot in it's common acquisition.