Those pokemon are event pokemon and the events are already over. The only way to get them is to ask some to trade in the Trading Forum or use an Actin Replay to hack the game.
The plates are for Arceus, they change what type of pokemon he is, like Grass or Fire type. But Arceus event hasn't come out yet in Japan or the U.S.
The polished rock is for Shaymin, the event hasn't happened yet in the U.S. but is already over in Japan.
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