The nearest airport are Toronto, Ontario in Canada. On USA side the airport is Buffalo, New York. Many visitor reach to New York airport and take 7 day bus tour of east coast including Niagara Falls.
Buffalore airport is only 25 miles from Niagara falls. Ontario airport is 70 miles from Niagara falls. You can rent a car, take shuttle to reach from airport to Niagara Falls. There are many hotels near to Niagara falls. Staying there gives you opportunity to visit Niagara falls at different times of day.