I have also been searching for it... 

The movie has actually been in Japan movie theaters since October of last year and has broken all kinds of records. You can find a cam of the movie pretty easily on torrent sites, but the quality won't be great. There still isn't an official release date (at least for the U.S. where I am) but hopefully it'll be out this year and better quality copies will make it online. 

I dont think there will be better quality ones. The demon slayer movie is only out in theaters, so i dont think you can watch demon slayer online, without it being in someones POV.

if your into torrenting. i believe there are a few decent copies with eng subs going around. but you'd have to find the file your self. i have seen it and its an amazing story and i believe will be canon to the series story line.