I was looking for Swedish numbers, which for some reason are not available under Phone Numbers in Thoughtly. I got help from Colin and Maksym Protsenko . I'm sure that they can help you too if you run into problems.
In my case Twilio misinterpreted things several times so it wasn't on the Thoughtly team that it took a long time.
If I was you, I would first check if Twilio at all provides Sri Lankan numbers. If they do, then start the process in the system to apply for a number and upload the necessary documents.
Instead of browsing around Twilio, you can use Google to do it for you... Place this in the search field and Google will return the indexed pages with "sri lanka" on them:
site:twilio.com sri lanka
