you will need a couple of things. first, a lot of these light guns are not very accurate, but you can modify a normal NES zapper to work with the Vs. system (that will be the cheapest solution).
The other part you need is the wiring harness (actually there are two of them). One hooks it into the control inputs of the PCB, the other hooks into the power of the cabinet.
So if you buy one, definitely check that it functions and how accurate it is as well as if it comes with the wiring harnesses.