You first get it in the cave before orebrough city by rock smashing after you beat the first gym. Later you are able to buy it in the Veilstone department store.
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In Pokemon Platinum, Flash is a TM.
all the Pokemon on dimmond that can learn flash can still learn flash in the platinum game too
how do you use flash? just go to pokemon and go to the pokemon that knows flash. press A and it will say in blue text "Flash" select that and it will use Flash. it's very useful in dark caves.
You shall have it when you defeat the 7th gym leader
Ok in the veilstone department store on floor B3 the top person sells it (it is TM 70)