Buy the Blu-ray to support the official release, then download the fan remaster for personal archival use. Many fans do exactly that.
The primary triumph of the 1080p remaster lies in the vindication of the show’s art style. The original broadcasts often suffered from "compression artifacts"—visual noise and blurriness that obscured the finer details of the animation. In standard definition, the show could occasionally look washed out, particularly during the darker, more atmospheric episodes of Book Two or the high-stakes battles of Book Three. The remaster eliminates this visual barrier. Suddenly, the intricate textures of the Water Tribe’s clothing, the distinct architectural differences between the Earth Kingdom and Fire Nation, and the delicate linework of the character expressions are crystal clear. The upgrade transforms the viewing experience from passively watching a cartoon to actively appreciating a piece of moving art. atla remastered in 1080p link
There is no official "native" 1080p source for ATLA because the show was produced in 480p. The best available version is the Remastered Blu-ray release , which is an upscaled 1080p transfer with significant color correction and cleanup. Buy the Blu-ray to support the official release,