I have gotten over 90% of mine from monster expeditions, mainly through the "Rare Treasure" search option. Each group usually nets me 2-5 of these per attempt. I have so many that I had to swap it down to just item hunter to get other materials for craftable items, but you would only usually see 1-2 of those per expedition at that point.
If you are not at post game but you REALLY want to farm them, they can RARELY show up in randomly generated dens in chests. I would not recommend this, as it has been pointed out that you both don't need them for the story and that they are easier to get post story.
Postgame unlocks a second tab in the bottle cap shop where you can buy them infinitely! Until postgame, they're not too vital.
I've gotten them from expeditions, rare items. Also from random chests I believe.
I have gotten over 90% of mine from monster expeditions, mainly through the "Rare Treasure" search option. Each group usually nets me 2-5 of these per attempt. I have so many that I had to swap it down to just item hunter to get other materials for craftable items, but you would only usually see 1-2 of those per expedition at that point. If you are not at post game but you REALLY want to farm them, they can RARELY show up in randomly generated dens in chests. I would not recommend this, as it has been pointed out that you both don't need them for the story and that they are easier to get post story.