Try looking here: http://www.en.customs.bg/index_en.html
It's all there, although just this once I have to agree with Scot's helpful advice, you will need a clearing agent. Even when I was working with Bulgarians importing stuff, they still used their own customs agent, to help "smooth the way" as it were. pm me for further info if you like.
btw, yes second hand clothes shops are doing well over here, lots of stuff from Germany. Have you ever wondered where all the clothes come from? Don't be suprised if all these secondhand clothes donated for charities end up being sold in shops across Bulgaria, possibly entering Bulgaria as a donation via a well known international charity, thus avoiding import duties!