Buy black pants. They should not be skin-tight, but they should not be baggy, either. If you already have pants like this, you can use them.
Buy a sky-blue, long-sleeved shirt. You also can use a white shirt. Even if you have a blue or white shirt, you might want to buy an inexpensive one for the costume, because you have to sew things onto it.
Sew black breast-pockets onto the cotton shirt.
Create a company patch, using an imaginary company, to sew on the right sleeve of the shirt. You can make the patch from felt and fabric paint, for example.
Wear a simple black tie, black shoes and a black belt.
Pin your fake badge above your left shirt pocket, and head out for the party.