It's pretty much common knowledge that the starters can only be obtained at the beginning of the game. The only other way is to trade them or migrate them to other games.
Gen I
Bulbasaur
Charmader
Squirtle
Gen II
Chikorita
Cyndaquil
Totodile
Gen III
Treeko
Torchic
Mudkip
Gen IV
Turtwig
Chimchar
Piplup
Gen V
Snivy
Tepig
Oshawott