The F-5 was made for export only. As a consequence, the US Navy never used it, while the Air Force and Marines bought some units in the 50's because they were affordable, but that was it. However the version present in the USA is not any version of the F-5, but a modified model, known as T-38. NASA and the US Air Force use the T-38 as a training aircraft, while third world countries all over the globe use it as their primary (and only, in most of them) jet fighter.