For most nations there are two options to get Sri-Lankan visa: ETA web-site (http://www.eta.gov.lk/slvisa/) or VOA (Visa On Arrival) in the airport.
If you’ve chosen to book your visa online on ETA web-site (http://www.eta.gov.lk/slvisa/), please consider a processing problem that still reportedly exists and may appear: sometimes the website charge money from your card, but doesn’t send any visa confirmation. In case of this you must contact your bank and block or dishonor the transaction.
To obtain a Visa On Arrival, you have to fill in the immigration card and proceed to Visa On Arrival desk located in the Arrivals Hall in front of the Immigration line. Visa application fee is 35 USD or 25 euros. After payment, take your receipt, immigration card, passport, and proceed to the Immigration line to get the stamp. Please note that sometimes immigration officer ask for return ticket and hotel booking confirmation.