It is a right faff to get the relevant linux-headers-* package for your Kernel version on a Raspberry Pi. I stumbled across a site that has packaged all recent versions of the kernel into kernel-headers-* packages.
They’re relatively easy to install, albeit slow. There’s a few dependencies but these are easy enough to resolve.
The files are at https://www.niksula.hut.fi/~mhiienka/Rpi/linux-headers-rpi/
In case this ever vanishes, I have mirrored all kernel versions at time of writing this post to here.