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In Viridian Forest but they are quite rare there. Or you can get them later at the Power Plant where there are many of them. But you need to have gotten HM01 Cut from SS Anne in Vermilion City and HM03 Surf in the Safari Zone. Either way if you missed getting a Pikachu in the Viridian Forest it will be some time before you can return there.

This site has a nice walkthrough:

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