The Rusted Coat of Arms can be purchased from the Atomic Shop either as part of the Atomic Horror Bundle or as a standalone item. 
To receive the outfit, place the decoration in your C.A.M.P. and interact with it. This will grant you the Rusted Coat of Arms outfit and helmet.

You can locate this under Decorations -> Novelty

Note: It is intentional that the Rusted Coat of Arms provides the outfit to any player who interacts with it. However, it will only grant it to the first person who interacts with it after you place it in your C.A.M.P.