Where can I officially complain about a Sri Lankan travel agency?

If you get hit by fraud or serious negligence, you can place an official grievance straight away with the Sri Lanka Tourism Development Authority (SLTDA) and also with the Sri Lankan Tourist Police.

The SLTDA maintains a specific complaints channel for international travellers, which is posted on its official web page. For urgent, on-the-ground situations, especially when there is harassment, theft, or a failure in a contract, the Tourist Police (you can reach them through local emergency numbers) have separate teams in key tourist areas like Kandy, Galle, and Colombo so they can provide quick legal help.