Scritps: https://github.com/fuchsia-mirror/jiri/blob/master/scripts/bootstrap_jiri
From this script we can know the download URL can be got from: https://fuchsia-build.storage.googleapis.com/
Example (end with commit id)
fx script: https://github.com/fuchsia-mirror/scripts/blob/master/fx
It's recommended to build jiri locally for now.