Costumes based on Thomas Magnum of "Magnum, P.I." are available from retailers, but you can save money by finding many of the pieces needed to capture this '80s look at thrift stores and online auction sites. In addition, putting the pieces together yourself allows you to concentrate on the details and get the look just right.
Wear a brown curly wig that's shorter on top and longer in the back. Glue on a fake, bushy brown moustache that covers the top of your lip.
Wear a red, short-sleeved Hawaiian shirt, preferably with green birds. Leave the shirt partially unbuttoned to show chest hair. If you don't have chest hair, create a fake one by cutting off hair from a wig and attaching it to your chest with fake eyelash glue.
Wear either regular jeans or small tan shorts that show lots of leg. Add a tan belt to either the jeans or shorts.
Put on white trainers or white deck shoes, and don't wear socks with your shoes.
Finish the costume with accessories. Add a black baseball cap; to make it more authentic, find a Detroit Tigers cap. Also wear aviator sunglasses. Complete your costume with a black toy pistol.