There really should be a clue as to where they might be. The Church one is easy, the one the Drug guy holds is easy, the spend 100k at Charlotte's is hidden, but you will get there eventually while fishing the gacha game. The Highland boss though.... its not hinted at, there is not description in the quests as of revision 1.0h, so you don't have a clue where to look. I'm making a guide for this...