* Undella Bay: You can buy TM Flash from the shop located on the left side of Undella Bay. This is the most common place to find it.
* Route 16: You can find a TM Flash on Route 16 by following the path towards the right side of the area. This TM is in a small cave near the entrance to the area.
* Pokémon World Tournament (Post-Game): You can earn a TM Flash by participating in the Pokémon World Tournament in the post-game. This is a more challenging way to obtain it, but it's a possibility if you're looking for a challenge.
It's important to note that you can only find a limited number of TMs in the game, so you might want to consider saving your money to buy it from a shop if you need it early on.
Happy hunting!