
While you are able to purchase Bronze, Silver and Gold Mother Boxes, also known as Loot Boxes, at will (and by completing certain tasks throughout the game), the same cannot be said about Platinum and Diamond Mother Boxes. Instead, you will only be able to obtain these Loot Boxes by accomplishing various (yet specifically set) activities in Injustice 2.
When it comes to Diamond Boxes, the best Loot Boxes and ones with the most items in all of the game, there are four ways to obtaining them:
1) Link To The Mobile Game of Injustice 2. The Mobile Version has a somewhat limited connectivity with the console versions, but after downloading it and access the Console Link option, you will be allowed to enter a code generated by a console version (found in Link to Injustice 2 Mobile in the Extras Menu). Doing so will net you not only a Grid Premiere Skin, but also the coveted Diamond Mother Box.
2) Complete Story Mode. After defeating Brainiac in Chapter 11 of Story Mode, you will earn a Platinum Mother Box. We repeat, a Platinum Mother Box. But after choosing either Batman's or Superman's side in Chapter 12 and completing the final chapter, you will be award a Diamond Mother Box.
3) Complete The Story Mode Alternate Ending. Defeating Chapter 12 will get you one Diamond Mother Box. But if you get back there again and finish the game by choosing the opposite side of the hero you chose the first time around, you will again obtain another Diamond Mother Box.
4) Complete Every Character's Tutorial. Tutorials alone are a great way for players new and old to the Injustice Series to become familiar with how to play, but you are also awarded for doing so. Complete one character's tutorial and you'll obtain a Silver Mother Box; finish the tutorial for all of the 27 original roster characters (minus Darkseid and other DLC Characters) and you'll receive one final Diamond Mother Box.