At present, almost the only way to get to Sri Lanka is a flight to the Bandaranaike international airport (CMB) to Katunayake, 30 km North of Colombo. The country has three more airports:
Ratmalana airport in Colombo (Ratmalana Colombo Airport), 15 km South of Colombo;
Mattala Rajapaksa international Airport (Mattala Rajapaksa International Airport), MRIA in the South of the island;
The airport in Batticaloa (Batticaloa Airport), opened on March 25, 2018 for civil traffic.
But all of them are not loaded, the flights are performed irregularly and work mainly on domestic flights, so for a foreign tourist Bandaranaike international airport is currently considered as the main one.
Many hope to resume the ferry from India, which existed before the outbreak of hostilities with the Tamils and was suspended in 1983. In 2011, an attempt was made to resume sea communication between Colombo and Tuticorin (India), but unfortunately, the new ferry company lasted only six months, and was closed due to financial difficulties. The war ended, but the parties did not come to an agreement, so the sea route with India remains in the ghostly future.
Air ticket prices remain fairly constant all year round. Basically, the earlier you book your flight, the more likely you are to get a good discount. Another way is to buy a package tour: flight + accommodation. Even if you do not plan to use the provided accommodation or use it only for a few days, the cost of the package may be cheaper than a single ticket.
Check the current prices for air tickets