One can't beat free content, and HDRI Haven truly provides a quality service that should cost so much more. The site notes 100% free, public doman HDRIs that go up to 16k resolution without having to sign up for anything. It's really amazing how such an open-source initiative can be so effective in the production of a lot of work. The site also has links to two other open-source projects and collections of resources: Texture Haven and 3D Model Haven. They also include options to help support these initiatives with donation through their Patreon pages.
3D Model Haven: