How to Find a Rock Star Candidate?

If you have an exact that is doing a reasonable job, but feel there is some one better out there for you or the company. How would you handle finding the replacement? If leaving immediately would cause short tern pain and create a hole in the orgWould you start a clandestine search? This seems hard to keep quiet and, quite frankly, a bit deceitful. Tell the exact you are going to replace them or level them.