Plutonium 244 is no longer generated in the world. This change happened a few versions back. I still get some that was generated in chunks created back then, but none otherwise.
Yep, I've only found 2 Plutonium veins on the official IT2 server (one manually, and 1 with an ender quarry). I do have piles and piles of plutonium though which I get from running uranium ore through a thermal centrifuge and also bees.
Sheldonite is Supposed to be located at between levels 40 to 50. Though I have not found that yet either.
I've found 2 Sheldonite veins on the overworld, again one manual and 1 with ender quarry. The best way to find platnium/iridium is to fly around in the End at the same height.