Most movies streamed are actually in Dolby Digital 5.1, and some are even in Dolby Digital 2.0 (basically stereo). The movie itself has to be encoded to have 8 discrete channels for 7.1 content. If you can’t find any movies on streaming, maybe buy or rent a bluray from this list:
https://forum.blu-ray.com/showthread.php?t=159814
I believe Netflix only has 5.1 content, but some stuff from Amazon Prime supports up to 7.1.