Where on Pokemon Diamond and Pearl how do you get flash?

you go to the cave where oreburg gate go up break the rocks and go from the stairs go down see at the right side will see a poke ball on the floor break the rocks at your right and go that place and take it. you will need in the cave near the cycling road exit.

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Alternatively, you can go to Veilstone City's store, go to the 4rd floor and buy it (TM70) for 1000 pokedollars