Pinwheel forest.
Cottonee can be found in Black in the Pinwheel Forest, Abundant Shrine, or the lostlorn forest. To get Cottonee in White you can either trade from Black, or go to the girl in Nacrene City and trade a Petilil (you can catch these in the Pinwheel Forest) for her Cottonee.
Cottonee's evolved form is Whimsicott. You get it with a sun stone.
I know there are others, but here are some examples: Pokemon Black-Gothita, Vullaby, Cottonee, and their families Pokemon White-Solosis, Rufflet, Petilil, and their families
Cottonee evolves by sun stone. If you have a sun stone you can evolve it at any time.
Black: Cottonee. White: Petilil
Cottonee does not evolve into Whimsicott at a particular level. You must use a Sun Stone on Cottonee to make it evolve.
Cottonee -> (Sunstone) Whimsicott
Sun stone.Of course its the same with COTTONEE,someone will be willing to give you a cottonee/petilil in that town...With leonora
For wild, catchable ones, it is Cottonee, Gothita and Vullaby. Cottonee however can be traded from an NPC in Nacrene City. Then as well as these, the legendary Pokemon Tornadus and Reshiram are Black-exclusive.
weedle, kakuna, beedrill, murkrow, houndour, houndoom, shroomish, breloom, plusle, volbeat, honchkrow, cottonee, whimsicott, gothita, gothorita, gothitelle, vullaby, mandibuzz, tornadus and (of course) reshiram.
Petilil, which evolves into Lilligant Cottonee, which evolves into Whimsicott
Cottonee is a Grass type pokemon.