Question

Where can you get TM Flash in Soul Silver?

Answer 1

Source for this

You get it in Sprout tower.
You can also buy it in the Goldenrod department store.
In HG/SS games it is TM70.

Hope This Helps :D

Answer 2

go to north in violet city(where 1st gym is)
You'll find a tower called "sprout tower"
go to the top, and defeat the elder Li