You can't do it.
Windows 7 Starter came preinstalled on Netbooks from the computer manufacturer such as Dell, HP, etc.
You could never buy it separately.
Windows 7 Starter came on cheaper OEM Manufactured Netbooks.
https://en.wikipedia.org/wiki/Windows_7_editions
Windows 7 Starter
Windows 7 Starter is the edition of Windows 7 that contains the fewest features. It is only available in a 32-bit version and does not include the Windows Aero theme. The desktop wallpaper and visual styles (Windows 7 Basic) are not user-changeable. In the release candidate versions of Windows 7, Microsoft intended to restrict users of this edition to running three simultaneous programs, but this limitation was dropped in the final release.^[8]^ This edition does not support more than 2 GB of RAM.
This edition was available pre-installed on computers, especially netbooks or Windows Tablets, through system integrators or computer manufacturers using OEM licenses.