You can not get PP ups in Pokemon platinum.
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You cannot buy PP Ups or Elixirs/Ethers in Pokemon Platinum. You can find them around the map, however.
You need to find them or buy them at Veilstone City.
The Energy Guru at the outdoor market in Slateport city sells carbos, zinc, iron,etc. but he does not sell PP ups or HP ups. Basically, you cannot buy PP ups or HP ups
You cannot buy them but im sure you can trade star pieces for PP-ups to a guy in Anville town on weekends
Nowhere. However, there are feathers that you can find on the Driftveil Drawbridge, which mimic pp ups, hp ups, zinc, carbos, etc.