Oh man, I was too fixated on your immediate question of where to get the kernel headers. Actually using DKMS with a dom0-provided kernel seems like it would be quite painful, because of the mismatched build environment (Qubes R4.1 kernels are built in Fedora 32) and probably a complete lack of integration with the distribution’s DKMS tooling too.
I’d switch the VM (the one that will eventually be running your custom kernel module) to a distribution-provided kernel like unman suggested - theoretically this should work for all virtualization modes, distributions, and VM types? - and then figure out DKMS in that VM('s TemplateVM).
Honestly though: If it’s just for some Wi-Fi dongle, your time might be better spent looking for a different one that simply works (at least with a recent kernel).