Привет ! Well...i can't really help you, because first i live in Sofia, second I haven't really used a broker firm ever. Most people avoid brokers as they charge 3-5% on all transactions. I did a search, and read one of the more trusted sites devoted to economics and housing. People there have given good opinions on this broker firm:

Недвижими имоти в Бургас

Open the link , this is the website of the recommended firm "Адрес" , it takes you straigth to the page with housings in Бургас. Browse, call the phone, and ask them questions. Other than that just Google, until you find something. if you need help with translating something i could help...although if you're buying in Bulgaria you probably familiarised yourself with the language.

I recommend foreigners to stick with brokers, sellers here always to try squeeze more out of Englishmen coming to live in Bulgaria, you'll be safer with brokers 3% more or not...

Why does anyone wish to move to Bulgaria anyway ? I wouldn't recommend that in the first place !