HM05 (Flash) is found on Route 2 in RBY. Requiring Cut, as well as 10 Pokémon registered in the Pokédex, simply head south from where Diglett's Cave is and use Cut on a nearby tree past the house. In the gate, Professor Oak's aide will be there where, as a reward for registering 10 Pokémon in the Pokédex, he'll give you HM05.
