Where to download firmware like 3.2 RC1, 3.2.RC12, so on?

Thanks for help in advance
I have been running APM 2.6 and 2.8 boards for a couple years and mostly always used firmware ver 3.2.1, or 3.1.5.
I am always reading about ver 3.2 RC12 or 3.2 RC 2 or other versions with the RC extension, where can a person download these versions with the RC extension?

You are a happy man.

RC is like release candidate, sort of test validation before final release.

So 3.2.1 is the definitive version for APM, following some RCx that you do not want to use (due to bugs or caveat).