N115 is taken. It's currently assigned to a 2004 Sikorsky HH-60L registered in Bozeman, MT.
See the full aircraft record →A tail number only frees up if the owner deregisters the aircraft or moves it to a different N-number — and even then the FAA holds it before releasing it to the public.
N115 has been worn before. Past registrations from FAA deregistration records: