Copy the font (ttf file) to ~/.fonts directory. Create the folder if it not there. You can use the command mkdir ~/.fonts to create the directory. Use ls -a command to view that the folder exists or not.
If you want to make the font available for all the users of the system copy the font to folder /usr/share/fonts
Run the command fc-cache -fv to load the copied fonts to the system.
Run the command fc-list :lang=si file to see what font files are available for language sinhala.
Restart your applications (web browser, etc) to make it effect.
~ Vidula Hasaranga - විදුල හසරංග ~