Aliens: Fireteam Elite

+ You can find hats, emotes, attachments in Hidden Caches through the game in each mission.
+ Each one will have a random assortment of items, typically three I believe it is, that you’ll get.
+ You do not have to finish a level after getting these caches and they respawn in random locations.
+ At the end of each mission completed you'll also receive a random assortment of rewards that can include any of these extras.
+The other option is to farm Requisition Credits and purchase Attachments from Sgt Park at the Armory in the Endeavor.
+You can use Challenge cards to help with this as they give you bonuses like x3 Credits for instance.
+Some of these Challenge cards will reward you with attachments and other items as well.