The only university department that specialises in teaching Bulgarian as a foreign language to foreigners is a part of Sofia University. It caters to the needs of foreign students and has a long-standing track record of academic excellence, although the majority of courses tend to be intensive and not easy from personal testimonies that I have heard. All foreign students admitted into tertiary education in Bulgaria are required to complete a prep year (may be a semester, depending on individual arrangements) there. They also organise courses for anyone willing to learn Bulgarian and may offer tuition either on a one-to-one basis or in small groups.
If attending school in Sofia is not an obstacle, have a look here http://www.deo.uni-sofia.bg/html_bg/bulfor.html