Support archives are generated on demand when requested, I don't believe they 'exist' anywhere permanently. This blog explains how to generate them:
That blog post is 2 years old. The support archive management has changed since then, and it appears they are now stored instead of just downloaded on demand:
I thought I was actually posting this (more recent) one with a similar name:
In either case, once you trigger the diagnostics it can store it within Cassandra but the supported way to access it would be to download it through the UI, not access it directly. Direct access to Cassandra is not supported.