Route 10,11 and 12 are the best for finding them
Find a Flying-type Pokemon and catch it. There are plenty of them in each game and many Flying-type Pokemon are common Pokemon, so you shouldn't have trouble doing so.
There are 22 flying-type Pokemon.
Swanna is a Water and Flying type pokemon.
Jumpluff is a Grass and Flying type pokemon.
Flying type Pokemon are good against normal Pokemon.
Crobat is a Poison and Flying type pokemon.
Flying Pokemon can appear in many places. (I know this is not a good answer but there's nothing else I can think of.)
The weakness of flying type Pokemon is Ice, Electric and Rock.
Thundurus is an Electric and Flying type pokemon.
Altiria is a flying type pokemon.
Tornadus is a Flying type pokemon.
The Pokemon taillow is a Flying Type