Well there's a few different Flying types available in Platinum, all have their merits.
These are a few of the flying types I have used and liked in my Platinum plays.
Staraptor is a strong flying type, with access to moves like Close Combat, Brave Bird, U-Turn, Steel Wing, and Double-Edge, paired with a 120 Base Attack stat it can be one heck of a powerful flying type.
It can be found in/on, Route 209, 210, 212, 215, Great Marsh, Lake Valor, Sendoff Spring, Trophy Garden, Valor Lakefront
Locations obtainable(As its pre-evo Golbat):
Route 227, Iron Island, Mt. Coronet, Oreburgh Gate, Ravaged Path, Sendoff Spring, Snowpoint Temple, Stark Mountain, Turnback Cave, Victory Road
The biggest problem with catching Golbat, and almost any Pokemon, at such a late stage in the game, is that it can be quite difficult to level them up for battling standards. So my suggestion would be to avoid as many trainers as possible before Victory Road, catch your Golbat, then go back and battle said trainers.
The other option you have is going and catching a low level Zubat and pouring the experience into him, this way evolving him into a Crobat becomes far easier that evolving a higher leveled Golbat.
Locations for Zubat:
Route 203, 204, 206, 207, 208, 209, 211, 214, 216, Iron Island, Lost Tower, Mt. Coronet, Oreburgh Gate, Oreburgh Mine, Ravaged Path, Wayward Cave
While this guy isn't as top notch as the first two he's still got a decent Special Attack stat and can use moves like Hydro Pump, Surf, Air Cutter, Shock Wave, Ice Beam, and Ominous Wind to get you through the game. As a water/flying type you can get him to learn Surf and Fly which are two useful HM moves.
Route 219, 220, 221, 222, 223, 224, 226, 230, Iron Island, Pokemon League, Sunyshore City
Togekiss has access to a great Special Attack stat at 120. With some great moves to back it up such as Air Slash, Wish, Ancient Power, Water Pulse, Shadow Ball, Psychic, Shock Wave, Fire Blast, and more it has the ability to be whatever you want it to be.
The only potential drawback to this Pokemon is that it is somewhat difficult to evolve. You will have to obtain a Togepi and from there evolve it into a Togetic with Happiness. Then you will have to obtain a Shiny Stone to evolve it into Togekiss. But if you do decide to go with it, you will have a very strong Pokemon.
Locations (as a Togepi):
Route 230, Eterna City
This Dragon/Flying Pokemon was one I enjoyed using very much. With access to both good Special Attacks and Physical ones it can do very well in-game. It also has a nice Dragon typing which is always fun to play with, and although there are better dragons out there Altaria still does very well. Some more notable moves are Earthquake, Fire Blast, Dragon Pulse, Ice Beam and SolarBeam.
Locations (as a Swablu):
These are a few good flying types in Platinum, and I would say pick whichever one you like the most, or whichever one you think will work best for your team. Hope this helps, and happy playing.