You can migrate one from Firered or Leafgreen.
You can get Pidgey by migrating from FireRed/LeafGreen or by trading with Platinum.
Pidgey cannot be caught on Pokémon Sapphire, so you will have to trade it from someone who has it or from FireRed/LeafGreen.
You must have a Pokemon firered or leafgreen trade you a pidgey then train it to level 36 and it will become a pigeot.
No one in the game trades you rattata nor pidgey if you want to trade those Pokemon use a link cable or wireless adapter to trade with leafgreen or another firered.
It elvolves from Pidgey at level 18 and evolves into Pidgeot at level 36.
Pidgey -> Pidgeotto -> Pidgeot 18lv. 36lv. If you don't understand, then the evolution is: Pidgey will evolve in 18lv. to Pidgeotto and Pidgeotto will evolve in 36lv to Pidgeot.
caterpie, metapod, weedle, kakuna, pidgey, pikachu, and i think pidgeotto might be in leaf green, but it is in Pokemon yellow
start game with charmender .catch a pidgey,pikachu,nidoran♀,geodude. raise this pokemons and after you have raise enough you should try to catch legendary Pokemon
you cant find a pidgey in Pokemon emerald the only way to get a pidgey is to trade with Fire Red or Leaf Green
find a friend with leafgreen who needs a rattata.
Pidgey evolves at level 18.