You might have to Bribe enemies for them. My suggestion is to either buy the guide book, they tell you what enemy will give you whatever for Bribe. Or, search around for a Bribe list.
If you're not that far in the game, then you might just skip it and come back to it later, if you get your hands on a Teleport Sphere, save it and once you have the Fortune Sphere, use it and you should be able to go back and get the Luck increase.
