TM70 Flash is in Sprout Tower, or you can buy it in the Goldenrod City Dept. Store.
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use the move flash
You are given a TM when you will need it.
Use the HM Flash this is given by the sage at the top of sprout tower.
Flash is an initial move learned by Volbeat in Generation IV. For the Pokemon who can learn Flash through TM70 in Generation IV, see http://bulbapedia.bulbagarden.net/wiki/Flash_(move)#Generation_IV_2.
buy another TM70 flas on the 5th floor of the Department Store in Goldenrod City.