There is (or was) open hardware and software out there; software has long been "open source" in the VR realm (ahem - REND386, for instance), and honestly Oculus and the Rift came out of the DIY VR realm.

You can homebrew everything you want today, assuming you have or can acquire the skills, materials, and tools.

If your skills are more on the software side, then it'll be harder. I don't know if it still exists (to purchase at least), but there is OSVR:

http://www.osvr.org/

Now - I know it looks "Windows Only" - but at one time, it did have Linux support, and the code is open source, supposedly:

https://github.com/OSVR/OSVR-HDK

But you can see that repo is somewhat old - I'm not sure what if anything is going on. But anyhow:

https://osvr.github.io/doc/installing/linux/

But then this:

https://www.tomshardware.com/news/razer-abandoned-osvr-joine...

Yeah - you'll probably have better luck homebrewing everything again. Dig out those old back issues of PCVR magazine, dust off your old copies of various DIY VR books, power up the 3D printer...

I would not recommend OSVR hardware at this time, but the effort for free software XR is still alive. With the recent provisional release of the OpenXR standard we at Collabora started making the code for our OpenXR runtime "Monado" public.

It's not entirely complete right now and relies mostly on reverse engineered drivers that mostly miss positional tracking, but we are hoping to have something better to show for it soon.

https://monado.freedesktop.org/

A short demo of the current status on a fully open source software stack using linux with amdgpu/radv, the godot engine and the libsurvive Vive driver: https://www.youtube.com/watch?v=eB-vVL-MPos