In Driftveil City, in the house to the west of the Pokémon Centre, you will find a woman who will ask to see a specific Pokémon with a specific move each day. Show her the Pokémon and move she has requested and you'll get a Heart Scale. This can be repeated each day.
They could've at least made it just a Pokemon, but no, we might have to waste a Heart Scale to learn the move to get a Heart Scale.
Remember when Luvdisc were wild? *remembers fondly*