Which specific release of L4T are you working with? You can find this by one of these commands:
head -n 1 /etc/nv_tegra_release
`dpkg -l | grep ‘nvidia-l4t-core’
This is available (if apt does not supply what you need; possibly the instructions you saw were for a desktop PC and not a Jetson) in source form for each specific L4T release. You can find your L4T release here (you might have to go there, log in, and then click the link again):
https://developer.nvidia.com/linux-tegra
(some content is available via apt-get in newer releases, but all releases are available as source from the particular L4T page)
Within this you will see something like this:
L4T Driver Package (BSP) Sources
This contains much more than just the kernel, but within this the “kernel_src.tbz2” has everything you need (including both in-tree and out-of-tree content of full kernel source). I typically copy the main source to a PC, extract just the kernel_src.tbz2, copy this to the Jetson at either “/usr/src” or “/usr/local/src”, and then unpack using “sudo tar xjf ...name...”. Note that you only need “sudo” if the unpack location is owned by root. I prefer this since I think do all compile steps as non-root. This guarantees my original source code won’t be polluted with configuration changes (having source owned by root, and building as non-root is a safety mechanism for future builds when you don’t remember what might have changed).
Result of unpack should show a “sources/kernel/kernel-4.9/” subdirectory. The out-of-tree content will be other subdirectories of “sources/”.