You can get all of the power items at the battle frontier. They cost about 16 battle points. Power items help your Pokemon GO faster in the process of EV training.
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There isn't a TM earth power in Platinum
The thorn anklet has no good use. All it does is reduce proficiency.
when you're making an anklet after you finish putting on the pixie dust on the anklet they'll ask you if you want to use the mermaid's voice
There is no Ancient Power TM in Pokemon Platinum, It can only be taught by leveling up, Move tutoring or by learning it from a Pokemon from an egg.
its a move your Pokemon might learn