Most villages in Bulgaria have houses with vacant rooms to let out to passing strangers, for very little money. The main problem is the lack of customers, so you will take them by surprise and the room will be unprepared, and perhaps even occupied by a family member, but they will soon sort it all out for you, and you will be made very welcome. If you have some basic Bulgarian, or perhaps with the help of a phrase book, just ask at the local shop or pub and you will be guided in the right direction.
Some will be more prepared than others. A family I know of in my village have the two rooms in their upper story beautifully furnished and vacant. They affectionally refer to it as the “hotel” and sleep downstairs (downstairs is warmer) on iron framed beds that are used as seats during the daytime. As far as I know they rarely, if ever, get any guests, but they hope that one day they will.
As Gimlet's post points out, these rooms are never advertised. To find them you have to ask around.