Trophy Garden. Beneath Hearthome City. You have to go into that mansion and through the door that'll lead to the garden. Run around.
In the morning, there is a 30% of finding one. During the rest of the day, there is a 10% chance.
To increase your chance of seeing one, put an electric type with the ability Static in the first slot of your party. It doesn't matter if it is fainted or not, it will still increase the chance of seeing an electric type by 50%. This should allow you to see mainly Pichus and Pikachus.