there are two ways to get flash. i cant remember the first one but you can get it at goldenrod department store for a few thousand poke dollars
It is in the sprout tower. If memory serves the old man you beat th the top gives it to you.
OR..
Go to Goldenrod city dept store the saleswoman is on the 5th floor it is called TM70 and costs $1000
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You could buy it at the Goldenrod City Department Store.
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.
no you can trade soulsilver pokemon to black
You CANNOT LEGALLY download Pokemon Soulsilver.
You'll need to trade for it from Pokemon HeartGold. It is not avilable in the wild in Pokemon SoulSilver.