If you already have the video files, you can search for standalone subtitle files (typically .srt format) on dedicated community databases:

A verified database for movie and TV show subtitles.

A focused site for English-language tracks. How to Use External Subtitles

In some regions, the show is available for purchase on Apple TV with original English audio and potentially closed captions. Subtitle Download Sites

The Series 1 DVD includes official English and Spanish subtitles. Note that some releases may be region-locked (Region 2/PAL).