So this doesn't directly answer your question, but if you're trying to find the .mobi files for use with Calibre, I would just download the Kindle for PC application and find the downloaded files for importing into Calibre.
They're stored in...
- Windows:
Libraries/Documents/My Kindle Content
- macOS:
~/Library/Containers/com.amazon.Kindle/Data/Library/Application Support/Kindle/My Kindle Content