The Office to Monitor and Combat Trafficking in Persons, the U.S. Department of State released Trafficking in Persons Report 2006 in June 2006. Section 5of the report - “Country Narratives” contains a subsection on trafficking in Bulgaria. 

You can find excerpts from the text for the Bulgaria section here or at:

http://www.state.gov/g/tip/rls/tiprpt/2006/65988.htm