Hi,
Re more precise administrative boundaries, please check here https://inspireportal.egov.bg/geonetwork/srv/bul/catalog.search#/metadata/c56d8289-c61c-4233-9803-fe30c6ba81de and here https://data.egov.bg/organisation/datasets/resourceView/f071d314-4cf1-45e8-9635-539257758d50