Aside from trading or receiving it on a gift Pikachu you may also learn it as an egg move. To have it as an egg move either have the male parent Pikachu know Volt tackle or have them both be Pikachu and have either parent hold the Light ball.
On Pikachu? (tags, always check them)
Well, You can trade with somone who owns a Pikachu with Volt Tackle (Hard to convince someone though..), or in D/P/Pt, you can get one through Pokemon Battle Revolution.
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