Platforms like Amazon Prime Video, Google Play Movies & TV, iTunes, and Vudu offer "Jack the Giant Slayer" for rent or purchase in high-quality formats, including HD and sometimes 4K.
If you truly want a standalone MKV, you’ll need to and rip it yourself using legal software like MakeMKV (for personal backup, where permitted by law). Intitle Index Of Mkv Jack The Giant Slayer