There are active Sinhala subtitle sharing groups (e.g., "Sinhala Subtitles For Hollywood & Bollywood Movies"). Join these and search within the group. Users frequently upload custom-timed SRT files.
You can visit the Baiscope Sinhala Subtitles website to find the specific subtitle file tailored for various versions of the film (BRRip, WEB-DL, etc.). Sanam Teri Kasam Sinhala Subtitle Download