I believe that the 6th (I think) Gym leader or at least one of the trainers in her gym has one.
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Drifloon is a Ghost and Flying type Pokemon introduced in the fourth generation. It can be found outside the Valley Windworks in Pokemon Diamond and Pearl.
Its drifloom in valley windworks
There's one simple answer: trade! Cranidos is to Diamond as Shieldon is to Pearl, so find a friend who has Pearl and trade with them.
There is no trainer in Pokemon Platinum with a Manaphy. However, you can trade with someone who has a Manaphy, or complete the special Ranger mission in Pokemon Ranger and transfer the Manaphy egg over to Pokemon Diamond, Pearl, or Platinum.
Where do you find a trainer who has a glacion in pokemon platnum?