You can find Flash, or HM05, in the house of one of Professor Oak's aides on Route 2 in Pokemon Yellow. Players must have at least 10 different Pokemon to get the HM ability.
To get to the house of Professor Oak's aide, players must travel east on Route 11 from Vermillion City. There, the player should stumble upon a small cave. This is Diglett's cave.
Wake the Snorlax blocking the cave entrance, and fight or capture it. Then enter the cave to access Route 2 on the other side. Players must encounter both Diglett and Dugtrio in the cave.
Once out, head south on Route 2. There is a row of two houses. Enter the second house and speak with Professor Oak's aide. He will give out an HM 05 (Flash) as long as there are 10 or more Pokemon in your collection.