Get S-RANK in these missions for them to be available at a shop in Extra mode:
You need to be at least Lv. 20 to equip them. Missions in Extra Mode are avaialble after you complete them in Story Mode.
Complete Chapter 4 in Story mode.
There is a way to unlock a special audio-only preview at the end of a (offline) story chapter. Each chapter has different timed missions, you MUST get a S rank with each of these missions to unlock the audio-only preview of the next chapter at the end of the chapter. Note: You also get to learn more about the characters within these previews.
When you are low on life find and you can't heal yourself, go to a save point and save the game. Then exit that game and reload it. You should then have all your health back!