Department of Immigration and Emigration of Sri Lanka: immigration.gov.lk -- provides official information on types of visas, list of countries whose citizens get can obtain visa on arrival, various visas available and visa fees information.
For the new tourist visa: eta.gov.lk
Migration Lanka Services: migrationlanka.com
Sri Lanka Customs: customs.gov.lk/passenger.html -- provides information and guidelines about customs regulations at entry and departure.