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 Rep Scrip and purchase Colorways 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.
+ If you have the money and these happen to be on sale, you can snag some dlc packs. There are four I believe and they come with 25 customize items each.

Per (E Vee Dub) Confirmed.
REQUISITION CREDITS Max = 50,000
REP SCRIP Max = 10,000

(I currently have 3,067 Rep Scrip, but another post said 10,000)

This achievement can be grindy depending on how many rep scrip you used buying challenge cards. If you're like me, and did just that, this solution can save you a lot more time (or at least some)

Although the game does not explicitly state it, any challenge card that gives a boost to credits will also boost your rep scrip by that same amount. Each difficulty gives a different amount of rep scrip, but that amount is the same across all missions only to increase as you go up in mission difficulty (casual, standard, intense, extreme and insane). If you, fellow achievement hunter, do not believe me, you can try it out for yourself. Do a mission on any difficulty, look at the end of mission rewards and look at the amount of rep scrip you get. The example I'll use is on standard difficulty, you get 50 rep scrip upon completion. Using a card that gives 2.5x credits will make that 50 increase to 125. Hidden caches do give some of the rewards for these cheevs, however, there is eventually a point where they don't give any more of so you'll have to use scrip to get more.