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Jacek Pilarski has recently shared a new batch of panoramic HDRI skies. You can now get 80 free 4K HDRIs from the developer's website.
The new free HDRIs feature sunny skies with clouds that are available in ACES and sRGB color space.
You can also buy higher resolutions of the HDRIs in case you need 15K or 30K for movie work. The developer's website also features a number of sets for artists looking for reference photos for matte painting and concept art.
You can find the full collection of panoramas here.