Greg Zaal writes:
The last time I posted here, there were "only" 53 free CC0 16k HDRIs available on HDRI Haven. Since then I've added a whole lot more (now 125), and also released over 700 backplate images for various HDRIs (also free) thanks to my generous patrons :)
All of the backplates are available in three different formats to suit your needs, including the original raw files, and are between 5k and 7k resolution.
All downloads (for both HDRIs and backplates) are done with the click of a button, no accounts or sign-up forms or anything like that.
The license for everything is CC0, meaning do whatever you want with it, it's public domain.
If you like what I do and want to keep the HDRI library growing, consider throwing a buck or two my way each month on Patreon :)