The FOSS version is here:
https://www.riscosopen.org/content/
It runs on all Raspberry Pi models up to the 4, and runs really well. As of version 5.30 it supports the Wifi chip in the Pi so it can connect to WLANs.
The owners of the IP are RISC OS Developments Ltd (ROD) and they have an enhanced version -- free on the Pi -- called RISC OS Direct, with an IPv6 stack and a modern WebKit browser:
https://www.riscosdev.com/direct/
They also sell more capable hardware for it.
There is another fork of the OS that is still proprietary. That's owned by 3QD. They used to sell upgrades for real Acorn-compatible kit but there's none of that that isn't vintage now, so it focuses on a software emulation that runs on modern x86 machines:
I have written about it multiple times, trying to spread awareness.
https://www.theregister.com/software/2022/06/21/risc-os-is-3...
Search the Reg for "riscos" and you'll find interview with the original core team, looks at the last couple of FOSS releases, the Moonshot effort to go 64-bit and more.