There are many conflicting websites stating that you need to apply for an ETA before going to Sri Lanka, and these websites then offer to help you get that ETA.
In reality, we went to Sri Lanka last week on an Indian passport. No hassles at all. Trip was planned 1 day before leaving – so no time for visa or ETA applications.
We were allowed to board the plane without the need for any visas.
We got a free visa on arrival at the visa counter at Colombo airport (just before immigration, behind the big Buddha statue).
Super easy. This visa was limited to a stay of 30 days.