OpenSubtitles: Offers a massive collection, though the site often contains heavy ads.
If your copy of the film doesn't have built-in captions, you can find them on several reputable subtitle repositories. Look for files that match your specific version of the movie (e.g., Blu-ray, WEB-DL, or DVD rip) to ensure the timing is synced correctly. Top Sources: harry potter and the halfblood prince subtitles