1080p/2160p Bluray remuxes would suffice. Oh yeah, you said websites. Private trackers are the best bet, if you want well-seeded torrents in super high quality. But *public torrents, from places like limetorrents and 1337x could suffice.

For specific films? Interstellar, Inception, Redline, The Tree of Life, Dunkirk, Blade Runner 2049, Vertigo, House (1977), Ran (1985), The Night of the Hunter (1955), Nausicaä of the Valley of the Wind (1984)

There's definitely many lists of great films to test of TVs/Projector setups, keep looking