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.

Effect - Code

Blitz Buster - Shrine of Fanatics (level 35 newdaiz)
Grome Buster - Mech Soldier of the Sands (level 40, moatoob)
Hegel Buster - Freight Train Rescue (level 15 train mission)
Raphaal Buster - The Remnants of Endrum (level 35 mission that starts from the subway)