I came across this just now doing the same search, and found the other answers are far too specific. I also google searched for downloading android-ndk-r8 and found next to nothing. To get the correct version, I instead went here:
https://developer.android.com/ndk/downloads/index.html
And copied the link to the download I needed, and pasted it into the URL bar. There, I edited the version to reflect what I wanted (for example, I changed r8b to r8). Then I pressed enter, and the correct download began.
As long as the naming conventions remain the same, this should work across all versions.
Edit: This convention did change. Some older versions are now available in the archives. For even older versions, refer to the links provided by the answer above.
