Time and effort. They don't magically appear. There is no webapp for it. You WILL have to spend a significant amount of time to find these people. It's probably also why you should be spending whatever you can to hold onto them once you have them.

You put out an ad? I would have assumed otherwise, but since you actually asked this question I'll make the obvious statement that a college degree is clearly no substitute for screening for 'smarts'. So either way, the ability to tell if someone is smart is still required. The degree is just more typically used as a crutch, imo.