Because BluRay releases like or x264 have specific frame rates, you need a matching subtitle file to prevent the text from going out of sync. High-quality subtitle sites include:
How to set the subtitle Language on a Blu-ray player. - Panasonic
: Widely considered the best for finding specific release-matched subtitles (e.g., matching the exact "YIFY" or "BluRay" tag).
: One of the largest databases with over 1.3 million subtitles in 50+ languages.