If you have a professional subscription, you can still download verified ISOs with integrated updates from the Visual Studio portal .
This popular third-party tool has a built-in feature to download official ISOs directly from Microsoft servers. Important Lifecycle Information Download Windows 8.1 Official ISO Image Files
Searching for a "Windows 8.1 Pro Service Pack 1 ISO" can be confusing because Microsoft never actually released a formal "Service Pack 1" for Windows 8.1. Instead, they transitioned to a "Feature Update" model, releasing a major cumulative update known as (KB2919355) in April 2014.
