This will be a bit of a challenge and require you to dip into the Coin field of the Gallery. There are roughly 65 bronze coins throughout the game, relating to the characters in general, and after that, the remaining coins are gold and come from special conditions (such as clearing specific Legend Diaries, completing specific events as specific characters, or from clearing Dream Log).
You will gain coins of any of your crew in a battle (allies) and any enemies you personally defeat (enemies) through most of the missions. Any secondary coins can come from meeting Treasure Event requirements or completing specifics to Legend Diaries. If you're running low on specific character coins, chances are you should run as that character (as you're guaranteed to gain more coins and Poster Experience by playing as that character rather than hoping for them to appear in Dream Log, for instance).
Personally, I received this as a mixture of working on the third Chapter and dipping into some of the Dream Log scenarios.