Then the short answer is there isn't source for these no longer supported (technically never supported as TV Boxes have never been officially supported) builds.
However, you seem to have sufficient technical skills and therefore I will point you down the path of building your own kernel (and thus you will have everything you need). Review the following thread: https://forum.armbian.com/topic/9625-compiling-and-booting-mainline-linux-for-your-s9xxx-tv-box
That is where I began my journey with armbian on amlogic TV Boxes. I think everything in that thread, (mostly thinking about my post with instructions) is still valid, although I have moved on to doing things differently now days. But I am running latest verisions of 5.4LTS, 5.10LTS and even 5.15.rc on various TV Boxes as I write this. So if you have the proper skills, you are not stuck with the kernel you currently have running.
