You can rent the official version directly through YouTube. Technical Specs to Look For
5.1 Surround Sound (the sound design during the elemental battles is incredible). Nezha.Birth.of.the.Demon.Child.2019.WEB-DL.1080... LINK
If you are looking for the best viewing experience, ensure the version you choose supports: 1080p (FHD) or 4K (UHD). You can rent the official version directly through YouTube
Another reliable source for a clean, high-bitrate digital copy. Nezha.Birth.of.the.Demon.Child.2019.WEB-DL.1080... LINK
Depending on your region, Ne Zha is often available to stream in high definition with both original Mandarin audio and English dubs/subs.