How do you get TM Sunny Day on Pokemon Black for Whimsicott?

Question

I want sunny day so that my whimsicott can learn it.for solar beam.

Answer 1

You buy it in Mistralton City's pokemart for $50,000 Poke-dollars.
The other TMs you can buy are:
TM07 (Hail)
TM18 (Rain Dance)
TM37 (Sandstorm)

Answer 2

You can buy it from the bottom clerk in the Mistralton city Pokemon centre