The best price found from United States to Sri Lanka is on January 30th, 2019, travelling with WOW air. See more options, compare prices and book: search flights.
Options for flying within Sri Lanka are very limited.
Bandaranaike International Airport has connecting domestic flights provided by Cinnamon Air. It offers a limited schedule of expensive flights to destinations that include Batticaloa, Dikwella, Sigiriya and Trincomalee. Service is on small planes, some using airforce bases, others with floats that land on lakes and lagoons. It also offers charter flights to points around the country at fares which start at US$3000.
Colombo Airport, Ratmalanae (www.airport.lk/rma), 15km south of Colombo, is an airforce base with a terminal that handles some domestic air charters.