If you have video footage or a DVD you want to convert, you can use video encoding software to create an MKV file. Encoding settings can be adjusted to prioritize quality.
Platforms like Vimeo or YouTube sometimes host Creative Commons licensed content. You can filter by Creative Commons licenses and look for high-quality uploads. index of mkv 300 high quality
Anything labeled "WEB-DL" below 10 Mbps or files under 4GB for a 2-hour film—those will crush the grain into blocky artifacts. If you have video footage or a DVD