It is found very late in the game. You have to navigate silk cave without it. Later in your adventure you will come to a port city called Castro valley. Go west and take Castro forest until it ends. Take the next route northwest and in the connecting building to the next route is an aide of professor Jen. He will give you HM05 Flash assuming you've caught 10 pokemon. The route above that is a lake. Find the only building there and the guy inside will give you HM03 surf, witch you can use after beating the Castro valley gym.
Chat with our AI personalities
you can find it as an item ball in the route before osuwari city
its not that there not there they are hidden in the top left cover will be the gym leader but u have to find him
The place where it flashes is where you can find the Pokemon It would flash blue if it was found in the water
Find an Abra to give flash in the cave in dewford.
Non. Flash is a HTM that you gotta find in the game!