I was not able to actually find a single pair of scissors until relatively late in the game. I managed to find two pairs in Obi's chapter. The first is in a room that you need to lockpick inside of a puzzle area where you must do a fuse box puzzle in order to open an electric gate to proceed. The second pair that I found was in the hospital/surgery section while looking for the surgeon's hand. I found a pair sitting on a gurney as I was exploring. I found a third pair on a gurney after climbing down the ladder marked with the red x on the wall.
If you don't have this achievement early on in the game, then don't worry. You'll be guaranteed to get it in the 'Obi' section of the game, as long as you pay attention and look for a pair. Scissors were also a 1 hit kill for me when I used them on a basic grunt.