HM flash is located in Viridian City. Once there you must go into Diglett Tunnel and walk around for a bit. You will eventually come across one of Oak's assistants who will give you HM flash as long as you have 10 Pokémon.
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To my knowledge i don't think there is a waterfall in Pokemon yellow and it is quite irrelevant to the storline if there was one. If you mean a HM named waterfall...i think your on the wrong game, only pearl and diamond have waterfall HM...i think. There are only 5 HM's in Pokemon Yellow Flash, Cut, Fly, Surf, Strength.
HM 05 is flash
In Pokemon Platinum, Flash is a TM.
There is no Rock Smash HM in Pokémon Yellow.
There is no HM Flash on Pokemon Diamond, I personally think its been replaced by HM05 Defog.