they tend to come from selling or using 3* and above in enhancements... I know 3* is worth 10, 4* i think was about 100, and i just found out 6* (used a dupe to get more equip slots and skill lv) is worth 12,000
if events aren't handing out 3* girls, go farm the mission for the rainbow bloom elixirs... as any non-elixir drop is a 3* girl you can sell for more bond crystals