The ADMX/ADML file can be found on Windows using the OneDrive client:
%ProgramFiles%\Microsoft OneDrive\<build number>\adm\

The OneDrive.adml file located in the root folder is the English file and must be placed in the en-us folder.

Sub-folders correspond to localized folders for each language. For example, the folder for French is named fr instead of fr-FR. Note that the folder names have not been renamed to match the ADML folder names used in GPOs, so you will need to rename them manually if necessary.

There are many versions available here, but this repo does not contain an exhaustive list of all ADMX files. I update the repository approximately once a month.