Where do you find the HM waterfall?

In Goldenrod City, you will first need to talk to the Game Corner clerk. This will be outside the building. If you don’t see the little kid standing next to the clerk you have found the wrong entrance of the Game Corner. Then enter the Game Corner and get one of the machines to spit out a Coin Case. Head to the department store and trade the Coin Case for the TM for Waterfall. It can also be found in Ecruteak City, in a house to the right of the Pokemon center as the 7th item in the top row of the cabinet on the right.