Luthor did in fact have quite a few stores of the element, along with all of the various varieties. Some in a raw form, others molded into bullets and weaponized in other ways. But the Joker did not need to know any of that.
"Illegal in the sense that it can drain the powers and ultimately kill an alien species," he snapped, already annoyed. "The mention of it brings about hard scrutiny and talk of being a traitor."
His eyes snapped to the Joker's face, understanding perfectly what the clown was suggesting. "You ... want to try to dispose of that alien," he said, the disbelief plain. "You," and the sneer formed as he stood, "dare to think it possible that you can take down that flying menace?"
There was a short silence and then the sound of loud, raucous laughter. But this time, the parties were switched, and it was Lex Luthor that was roaring with laughter. "Y-You?! You can't even take down that human shadow, after HOW many years of trying?" The laughter grew louder, Luthor actually having to lean on his desk for support. "And, and now you dare think that where others, more powerful and brilliant than yourself have failed, you'll succeed? HA!"