Pokemon sapphire how to get the HM flash?

You can't get the HM Flash in Pokemon Sapphire.

Here's why:

* Flash is exclusive to Pokemon Ruby and Emerald: It's used to illuminate dark caves, and is a key HM in those games.

* Sapphire focuses on a different set of HMs: It emphasizes different gameplay mechanics, and Flash isn't part of that.

If you'd like to experience Flash, you'll need to play Pokemon Ruby or Emerald.