Is there a repository/site or any update package for the administrative templates in the GPO (Group Policy) of the MS Active Directory?
You can create a central store in SYSVOL on a DC and keep all of the GPO templates there.
Here's a good KB article on creating the central store: http://support.microsoft.com/kb/929841
Whenever a new IE, Office, OS, or GPO enabled MS product comes out, there are templates for it available though admin packages, or right in the Windows folder in the case of the OS. They are almost always available on the MS download site. If you add these templates to the central store, they will be available to anyone that tries to manage GPOs via the GPMC in your domain.
It's not exactly automated, since you have to copy the new templates over yourself, but it ensures that all workstations using GPMC are working with the same templates.
Below is some good background information on Admin Templates. I do not believe MS provides a central respository. You probably need to go to the Product information for varuous prodcuts and locate them there. Google is you friend for that.