It was likely removed, but in one of the previous versions of the game, in the cave while you're chasing Professor Oak and killing all of those people, it was possible to encounter starter pokemon such as Charmander. Believe I saw a Pikachu once too... But again, that was in a previous version, where the second post game chapter was the end of the game.
I can't find one either. I gave up and just caught other Pokemon instead. And there is a way to get Squirtle as well as Bulbasaur if you choose Charmander.
SPOILERS
Once Professor Oak has left his lab, you can steal Bulbasaur's Pokeball from the table it was left on. As for Squirtle, you need to defeat your final opponent in the Final Dungeon's third floor.
