I tried searching the forums but didn't come up with anything. I've not been able to find the answer to the question before and I figured this is the place to ask.

buys it off people who find it, or once found he hires a crew and mines it himself.....

Apparently this rare ore from an extinct planet is really easy to find because it seems like everyone has some.

Pre-52, Luthor got hold of it from Metallo. Metallo in turn, had been resurrected by a scientist who had found a fragment of Kryptonite. This took place in Byrne's Man of Steel, I believe.

In the New 52, Luthor (to my knowledge) does not have possession of Kryptonite yet. Though it does already exist in this continuity.

He's a rich crafty bald bastard, he has a place.

In the new 52, he stole some from the military.

For how he got it in the new 52 read action comics annual 1 and for how he got it pre 52 there's three versions cuz there were three origin stories. Man of steel volume 2, birthright, secret origin.