Question

Where can you get TM Flash in Soul Silver?

Answer 1

Non-repeatable
- Sprout Tower (Defeat Elder Li)

Repeatable
- Goldenrod Department Store 5F ($1000)

TM70 (Bulbapedia)

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