Streams from Amazon Video. It’s a buck cheaper for the SD version, just click on more purchase options.
This seems like a very good price.. It was just released this month in the US.
Here is link to info about the movie from Funimation’s web site but please note that this deal is for digital version and not DVD/Blu-Ray combo.
https://www.funimation.com/shop/home-video/assassination-classroom-365-days-movie-combo-bld-00722/?
Here is a link to a trailer video for the movie.
https://www.youtube.com/watch?v=boi959T
https://www.amazon.com/dp/B07FGG59Z3