i was trying to be vague, but probably someone somewhere will rip them and made available for download somewhere
reading around it seems most people have luck using amazon fire tv stick, or a similar amazon product. It doesn't seem to be available on the amazon app via roku, google tv, apple tv etc. My guess is it'll roll out on those later.
however, I just did a small test. using microsoft edge browser, I logged into my prime account, and played s6 e1 from the browser. then I paused it, and in the top left corner there should be the x-ray and then you can select the extras. This might be possible on other browsers but didn't try.