You can buy them at Lix, the town surrounded by mountains, and filled with trees. You need to be able to get there by the air, so for sure you need the Airship, and might be able to get there with the Black Chocobo. As I recall, there's a Piano there, and you can get a song there as well, but at any rate, be thorough with this town because events will make this place inaccessible later.