In both versions, you can get them in Nimbasa City, Relic Castle, and for Black version only, Black city for 10,000 money. You can also get it by using a Pokemon that has the ability pickup and is over level 31, and you can get sun stones in dust clouds.
Black: Cottonee. White: Petilil
You use a Sun Stone on it.
Cottonee evolves by sun stone. If you have a sun stone you can evolve it at any time.
With a sun stone. a Sun Stone can be obtained in Nimbasa City and Relic Castle.
Only Cotonnee (black) and Petilil (White) can be evolved using the sun stone in Black and White. However, you can catch Gloom and Sunkern after obtaining the national pokedex, or you can simply relocate from previous generations.
find a sun stone.
You can find a Sun Stone in Raimon/Nimbasa City (The 4th Gym City) The Sun Stone can be obtained from a person in the apartment left of the Battle Subway on the 2nd floor.
Cottonee does not evolve into Whimsicott at a particular level. You must use a Sun Stone on Cottonee to make it evolve.
Petilil will evolve with a sun stone, pansage with a leaf stone, panpour with a water stone, and pansear will evolve with a fire stone.
Cottonee's evolved form is Whimsicott. You get it with a sun stone.
The Mythbusters found that a white car is cooler in the sun compared to a black car.
Dusk stone: Lampent Fire stone: Pansear Leaf stone: Pansage Moon stone: Munna Shiny stone: Minccino Sun stone: Cottonee and Petilil Thunderstone: Eelektrik Water stone: Panpour