It really depends what you are looking for and what your budget is. The properties to look for will be different, the places to look will be different. Village house, apartment in town, beach resort, mountain resort? How much renovation work are you willing to consider, or do you want something ready to move into? Is it going to be a holiday home, or do you hope to move? How much do you want to spend?
For village houses, as @JimJ said, check out the Facebook seller groups, there can be some reasonably priced village properties there with Brits wanting to sell up and move back to the UK. There are also some way overpriced properties there, or properties that look good in the five year old photos but have serious issues now. But it will help give you an idea. You Tube, too. Rightmove shows listings from multiple sellers, including the two longest established pay-monthly sellers, Bulgaria Direct and Mowlem. Both have good reputations, offer good after sales support, and mostly sell village houses. Unless they specifically say otherwise, assume that the properties they sell will need significant renovation work. Bulgarian Properties is a Bulgarian based online site listing a range of properties, probably not bargains there, especially when their minimum 2000 EUR fee is added on, but they do have a good reputation among some forum members, and have a lot of apartments and resort properties as well as town and village houses, if that's what you're looking for.
If you haven't yet, do try to visit Bulgaria to get a feel for the country and look at a few places to see the condition of properties in your price range. I'd visited six or seven times, including staying in villages, and thought I had a fair idea what to expect. I was still surprised at the amount of work my main village house needed. I bought two, via Bulgaria Direct using pay monthly. One was super-cheap and I knew was basically uninhabitable. The other house, costing more but still very much at the budget end of the market was more or less livable (less!), and in need of extensive renovation. I certainly found Bulgaria Direct easy to deal with, the pay-monthly and buying process was clear, the contract is robust, everything was done according to the law, and the support from their admin was excellent.