It is very hard because it is a starter Pokemon and the only starter you can get is pekachu so trading from another game is the best idea.
That is what I found as the answer before, but if you're playing the Lightning Yellow that I am, you can only catch Bulbasaur in one patch of grass in Cerulean City. It's right below a house.
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You can get bulbasaur at this lady's house in Cerulean City. She is a Pokemon nurse who catches Pokemon and nurses them back to health
Trade it from yellow/red/green.
A woman in the house West of the Cerulean City Pokemon Centre will give you a Bulbasaur if your Pikachu is happy enough. The Bulbasaur is level 10.
Once Pikachu is happy with you, talk to the lady next to the gym in Cerulean City. She will give you a Bulbasaur.
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