You can buy TM70 Flash at the Veilstone Department Store.
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You can get Flash as TM70 from the Elder in the Sprout Tower. Additionally you can buy it with your in-game money in the Goldenrod Department Store.
use the move Flash (TM70)
In Soul Silver, you can find Flash (TM70) in the Sprout Tower and in the Goldenrod City for the price of 1,000
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.
You can find TM70 Flash in Oreburgh Gate, the cave west of Oreburgh City. Once you're inside, go left, then go up, then right. Go downstairs. You will need to use Rock Smash in here to find it. You can also buy it on the 3rd floor of Veilstone Department Store for 1000 Poke.