The new masks are found in the armored PMC boxes that are guarded by FDI troopers. There are two places to get this achievement. The first place is shortly into Chapter 8. You will come across a PMC box and kill the guards before they can close it. When you search the box (Press ), you will find a mask and the achievement will unlock. You must be playing as Elliot Salem to search this box.
The second PMC box is encountered towards the end of Chapter 9. It is found when you rescue the final two hostages and you can be either Salem or Rios this time.