Mobile data in Sri Lanka is relatively cheap the 4G coverage covers most parts of the island. Dialog and Mobitel are the two prominent mobile network providers and both offer special packages for tourists. You can buy a sim card from an outlet of either company at the arrival lounge in the Airport.
Dialog Tourist Plan – https://www.dialog.lk/tourist-plans
Mobitel Tourist Plan – https://www.mobitel.lk/prepaid/tourist-pack