https://www.videohelp.com/software/x264-Encoder
There is a lot of different builds of x264 and x265, with different compilers and optimization options. It is not always easy to grab the best one, but currently, I trust VideoHelp. It links to download.videolan.org anyway.
BTW, you can easily open the download page of any external tool distributed with BD3D2MK3D with Help menu -> "[toolname] version".