Happiny is one of the more rare Pokémon to find throughout Pokémon Brilliant Diamond and Shining Pearl. This Pokémon evolves into Chansey if it is leveled up while holding an Oval Stone during the morning or daytime. Chansey will then evolve into Blissey if your friendship level is high enough. Here is where you can find Happiny so you can get Blissey later on.
Before you can get Happiny, you will first need to reach Hearthome City. This journey will take a little time, and you will first need to defeat the gym leaders in Oreburgh City and Eterna City. Once you have defeated these gym leaders, take the bike path south from Eterna City and head east to reach Hearthome City.
Once you reach Hearthome City, head east to find a hiker standing next to the Route 209 sign; talk to him to get an egg. This egg will take 10,000 steps to hatch. Luckily, you can keep track of your steps with the pedometer on the Pokétch app. When the egg hatches, you will have a Happiny.
You can also find Happiny in the wild by going to the Trophy Garden. You cannot access the Trophy Garden until you have unlocked the National Pokedex after defeating the Elite Four.