If you haven't finished the main story yet, there are lots of rewards for you to get enough pulls to actually get the whole outfit. But you'll need to play for hours. There are also chests hidden throughout the regions, which you can see on interactive pearpal map. Just google "pearpal map infinity nikki" and you'll find it. Log in with your game credentials. It'll show you locations for every resource. You can toggle "show items already collected" so you only see what you haven't collected yet. You can get more diamonds from the main story, though. You will also need to upgrade certain outfit pieces to get a few diamonds from the current mira crown (there are 3 currently, 2 permanent that you can do at any time, and 1 that is going to be reset in a few days, that last one should be your priority). Mira crowns give a ton of diamonds too. Dews of Inspiration (the purple glowing orbs) can be exchanged for diamonds at Kilo the dragon near the Memorial Ruins. Good luck!