Thanks for your lists. Unfortunally we have not yet found free/open polygon information for the Serbian admin divisions. Normally the national mapping agencies or statistical offices provide admin boundary polygons.
The former has a gis portal with boundaries, but I am not sure about the license and download options.
Let me know if you can find the boundaries. As it will allow to automatically assign the admin codes to the features when within the boundaries.
Shape files are good, they are a specific file format for geographic polygons. However, diva-gis is somehow reladed to gadm.org which has a restrictive license for academic use only and we are not allowed to use it.
The good news is I have found boundaries in Natural Earth which has a public domain license. The update will be in tomorrows export.