http://lmgtfy.com/?q=headset+trek+500
Oh, look! March 3rd, 2014 a headset question about a 1986 Trek 500! It's by some bikeforums user named Pars? Weird.
A headset is $40 The bike shop has one. My guess is it's a standard 1 inch threaded. If the top nut threads on easily with fingers, you've got the right one.