Where do you get flash in Pokemon Black and White?

You can't actually get Flash in Pokemon Black and White. Flash is a HM (Hidden Machine) that was introduced in Generation II (Gold, Silver, and Crystal) and was used to illuminate dark areas.

However, in Pokemon Black and White, the developers replaced the need for Flash with "Light Stone", which is a key item that you obtain early in the game. It automatically illuminates dark areas, making it unnecessary to learn a separate HM.

You'll get the Light Stone during your journey, and it's not a difficult item to find.