In Nimbasa City
The house directly above the Pokemon Center has a lady inside that will determine how happy your lead Pokemon is with you, while the Ace Trainer right near her will give you a Sun Stone if you talk to him. That can be used to evolve Sunkern, Petilil, or Cottonee. Then, go to second floor of the house next to that and talk to the Socialite and she'll give you a Soothe Bell if your lead Pokemon is at least moderately happy with you.
^Well half of that had nothing to do with this. xD