While I cannot provide a live direct link here (due to evolving URL changes), the safest mirror is usually hosted on AndroidFileHost under the user account "Androxyde". Search for "flashtool-0.9.28.0-windows.exe" on AndroidFileHost.

Flashtool is the de facto standard for managing Sony software outside of official channels:

Provides tools to unlock/relock bootloaders, which is often required for custom ROM installations.