Certain Pokémon are only available through breeding, and some of the returning faces in Sword and Shield’s Isle of Armor expansion are going to be a little tricky to get if you don’t know the proper method.
While Azumarrill and Blissey’s evolutionary lines are both obtainable in Sword and Shield now, the baby forms of each Pokémon require a special form of breeding to obtain.
Much like when trying to obtain Munchlax by breeding Snorlax, you will need to head over to the Hulbury market and talk to the Incense saleswoman. Once there, you will need to purchase the Luck Incense and Sea Incense.
You can then take your Marrill or Azumarrill and your Chansey or Blissey to one of the two Daycare Centers available in Sword and Shield. Give the Water-types a Sea Incense and the Normal-types the Luck Incense before placing one of them into the Daycare with a compatible partner, it is most optimal to do this with a Ditto to guarantee the species you want.
After traveling around for a bit, head back and collect the Egg left by your two Pokémon. Upon hatching that Egg, you should receive either an Azurill or Happiny depending on which Pokémon you placed inside.
If you don’t use the Incenses while breeding, you will instead receive a Marill or Chansey, so make sure you give each Pokémon the right item.
Happiny and Azurill could still be available in certain parts of the Isle of Armor as random encounters, but the best way to get them and add them to your Pokédex is by using this breeding method.