For Windows users, the Microsoft Store is an excellent resource. When you buy a movie, you have the option to download it for offline playback. The video quality is generally on par with Blu-ray standards, making it a reliable place to for PC viewing.
Apple is widely considered the gold standard for digital movie quality. While they market their highest tier as "4K," the files are often downscaled beautifully to 2k/1080p depending on your device, maintaining incredibly high bitrates. Their "Download" feature allows you to save movies to your hard drive for offline viewing via the TV app. Download 2k Movies