You cannot buy a PP up in Pokemon crystal! You need to search for it in caves and other areas you might think it's in! Buy the guide booklet to know where items, Pokemon, and get to know gym leaders or pokedex!
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You can't. You can only find a PP Up
You Can't
You cannot buy PP Ups or Elixirs/Ethers in Pokemon Platinum. You can find them around the map, however.
You cannot buy them but im sure you can trade star pieces for PP-ups to a guy in Anville town on weekends
They arent for sale, you can only find some.