Hi Ian!
Thanks for reaching Microsoft Community. We're here to help and let's work on this together.
Unfortunately, here in community forums we do not have access to the W9 form for the Microsoft Store. However, you can contact the Microsoft Store support team directly for assistance with obtaining a current copy of the W9 form. Please see through this link Contact Microsoft Store Support - Microsoft Support on how to contact them.
Hope this helps!
Regards,
RodelioP.