To become an NRA-certified Instructor, you've got to take the appropriate NRA class. So far, so obvious, right?
The people who are allowed to offer the Instructor class are called Training Counselors. And that's the rub... there are only a handful of us in the state, most of whom are on Oahu. While a single TC can offer the class, it's both a PITA to do, and not encouraged by the NRA. Also, as zippz said, it requires 2 1/2 days, which does not include the mandatory screening process. Before you can be enrolled in an Instructor class, you must meet with a TC, demonstrate both safety and proficiency with a variety of firearms (in the case of Pistol, these include a semi-auto, single-action revolver, and double-action revolver), and pass a marksmanship qualifier.
The upshot is, the classes tend to be pretty far between, and availability is limited. This is why some people travel to the mainland rather than wait.