because you cannot fly out till after christmas day you will probably only have the option of booking flights and accomodation separate, as far as I know all flights for tour op holidays only travel on Sat or Sun each week, there do not apprear to be any mid week flights for bulgaria.
jason try BA for flights they seem to go everyday to sofia, although a lot of accomodation is from sat to sat but if you put in google you will find a site with loads of private lets and I am sure if they are free for the time you want to go you would have no problem.