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.