You can find a Pikachu at the Viridian Forest. You have a 1% chance of encountering one normally, so I would recommend having a Pokémon with the ability Static in the front of your party. The Mareep line can have this ability, so you should catch one if you haven't already and have it in the front of your party. This will make Pikachu a lot more common.
Source: knowledge and experience
