Yes. Human Rights Watch has been able to conduct a lot of interviews since the conflict ended, and while some of these have taken place inside the country, a lot of them have taken place outside.
There are people who you're able to interview. For instance, even in the Channel 4 documentary, The Killing Fields, they interviewed a number of members of the Sri Lankan military who have now left the country. So there are certainly other Sinhalese sources who you would be able to interview.