An HDRI is one file that decides the color of every shadow in your scene. It is the highest leverage asset in a 3D project and the one people spend the least time choosing.
#Where to get them
Poly Haven is the answer, and it is not close. Hundreds of skies and interiors, shot properly, every one CC0, in resolutions from 1K to 16K. No attribution required, though the project runs on donations and deserves them.
ambientCG has a smaller set, also CC0, worth checking when you want something Poly Haven does not have.
Beyond those two, be careful. HDRIs turn up in mixed-license collections and repackaged on sites that did not shoot them, and a 3 GB file with no provenance is exactly the thing you do not want in a commercial project.
#Resolution: less than you think
For lighting only, 1K or 2K is usually enough. The HDRI is being blurred into an irradiance probe, and the detail is thrown away.
You need the big version only when the sky is visible in the background as a sharp image. Even then, 4K is often plenty, and 8K and 16K are for stills rather than a game that has to load in a few seconds.
The practical approach is to light with the small one while building and swap up only if you can see the difference. Most of the time you cannot.
#Choosing one
Three things matter more than how pretty the preview looks:
- Time of day, which sets the direction and length of every shadow. A midday HDRI gives you short hard shadows and a flat look.
- Cloud cover, which sets how soft everything is. Overcast is forgiving and can be dull. Clear sky is dramatic and unforgiving of geometry problems.
- Color temperature, which your whole palette then has to live with. A warm sunset makes blues fight you all the way through art direction.
Pick the HDRI before the palette, not after.
#Keeping them findable
HDRIs are big, they have unhelpful names, and they are impossible to tell apart in a file manager, which is why most people end up downloading the same one twice.
Tessera previews them as panoramas rather than as a row of file names, so the library is browsable the way the Poly Haven site is, offline and alongside everything else you own.
Install Tessera, or read looking closely.