Edit: The costume can only get obtained if you choose to go to the Ag Center. If you choose to go to Highpool the Ag center will no longer be available to visit. And as such you will not be able to get the costume, or the achievement
I personally havent got this achievement yet, but have found the costume needed to unlock it. At best guess, you dress one of your characters in this costume and leave it on them until you complete the game.Edit: you dont need to wear the costume the ENTIRE TIME, just at the games ending
The costume is located in the east field of the Ag Center, inside a safe in a stack of broken bottles. You will need perception (Just one point of perception is needed. Thanks to A TROLL AHH for the info!)
to see the safe and lock picking to open it.
The supplied screenshots show the in game location of the safe and the map location.
Cheers.Edit: Because of some tears in the comments below. Not a good idea to wear the suits in the area called the "Canyon of Titan". There are enemies which do explosive damage which destroy the clothes you are wearing. And once destroyed they are gone for good.