You can get it in the goldenrod department store.
During the bug contest at the national park if you see a scyther catch it; it has false swipe, and cubones also learns it.
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false swipe is learned by the following Pokemon upon levelling:scythernincandazangoosefarfetch'dgalladegrovylecubonehowever, false swipe is also a TM. this makes the move available to other Pokemon as well (kriketune, weavile, parasect, kingler, anorith, etc).
To teach Smeargle False Swipe you need to find a Pokemon to fight that knows False Swipe. Make sure Smeargle has at least one copy of the move Sketch. During the battle with the Pokemon that knows False Swipe use Sketch after the opposing Pokemon uses False Swipe. If done correctly your Smeargle's Sketch should permanently turn into a False Swipe.
After you've seen 25 Pokemon or more, talk to Professor Juniper and she will give you the TM for false swipe
The Pokemon tha has false swipe in Pokemon diamond is Shedenja
scyther at least.