According to the Law for Foreigners’ Stay in the Republic of Bulgaria, each foreigner may enter the country with a valid passport (or other ID document) and an entry visa for Bulgaria. Entry visas are issued in all Embassies or Consulates of Bulgaria abroad. No visas are required for citizens of the countries of the European Union and of a number of other countries as well. On arrival in Bulgaria every foreigner, if not accommodated in a hotel, should, within 24 hours, register his/her address with the Passport Service for Foreigners.
Site of Ministry of Foreign Affairs with information on visa application: