Where and how can I earn Heart Scales?

If you don't know this is cost for one usage of Move Relearner (in Dendemille Town) skills.

Fish with the old rod at Route 8, 12, Ambrette Town, Azure Bay, Cyllage City in Pokemon X or Y. You can steal them from Luvdisc with a 50% chance of them being held.

Or you can get one daily from Tierno in Couramine City after beating the Elite Four, if you have a Pokemon knowing the move he wants to see.

I had stolen about 10 heart scales in a matter of minutes.

Edit: If you want to kill 2 birds with one stone, use a Pokemon with the move called "Covet". Not only will it steal the item, but it will damage the target pokemon in hopes of fainting it to gain experience.

Using a Pokemon with the ability Frisk is the best thing to do.My Trevenant allows me to easily find Heart Scales.