Question

What are all obtainable Flying type Pokemon in Platinum before battling Gardenia?

Answer 1

Flying types prior to Eterna Gym:
Gyarados (much grind)
Starly
Staravia
Staraptor (much grind)
Zubat
Golbat
Crobat (much grind)
Beautifly
Butterfree (evolve from Caterpie, requires dual-slot with FireRed version)
Drifloon
Drifblim (much grind)
Hoothoot
Noctowl
Chatot (in-game trade)
Combee (Honey Tree)
Vespiquen (much grind as it will be difficult to get a female Combee)
Mothim (evolve a male Burmy from a Honey Tree)

Personally I recommend Golbat, or Crobat if you can max its happiness super fast.

Comments

Gligar requires a bike, so it's not obtainable before Gardenia.

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I added the Pokemon you can get from Honey Trees, as Honey is obtainable after defeating Team Galactic in Valley Windworks, which is before you reach Eterna City

Answer 2

You can get Starly, Zubat, Hoothoot, Beautifly, Drifloon and Gligar.
Beautifly is a 1% encounter, so catching a Wurmple would be easier than hunting for it, and Drifloon can only be found once every Friday.

Source: Searching on Bulbapedia.