Diamond and Pearl: Where to Find Drifloon
Drifloon is #065 in the Sinnoh Pokedex and is one of the Pokemon that many people need to encounter to finish their PokeDex after defeating the Elite Four.
It must be Friday and you must have already beaten Mars at Valley Windworks in order to see a Drifloon.
On a Friday during the daytime, fly to Floroma Town. Now head right from the PokeCenter until you get to Valley Windworks. You should see a Drifloon in front of Valley Windworks. You will not encounter Drifloon by walking through grass, you will only encounter one by finding it in the overworld and pressing A to initiate the battle. Drifloon will always be a level 22 female.