Nationals from Maldives Seychelles and Singapore can enter without a visa and ETA/Visa.
Everyone else can apply for an ETA (Electronic Travel Authorization) prior to arriving. The ETA costs US$25 for SAARC countries and US$35 for everyone else. Sri Lankan visas can be extended for up to three months without much difficulty.
Nationals from Cameroon, Nigeria, Ivory Coast and Ghana will have to find a sponser to apply for the ETA.
ETA Sri Lanka
Sri Lanka Embassies