What market or poke center is the place that a man gives you flute to awaken regigigus in Pokemon platinum?

The man who gives you the Flute to awaken Regigigas in Pokemon Platinum is not in a market or Poke Center. You actually find him in a cave called Snowpoint Temple which is located in Snowpoint City.

He's inside the temple, and after a short conversation, he will give you the "Azure Flute" and explain its purpose in awakening Regigigas.