If you are considering a dog, then you should really consider adopting. Street Hearts seem to have a steady supply of dogs looking for a loving forever home #adoptdon'tshop
Although not actually based in Sofia, Street Hearts is an independent Non Government Organisation that works alongside the municipality of Dryanovo to try and reduce the street dog population in the region. They rehabilitate abandoned or poorly dogs, train them and look to find them homes. All puppies that they find are kept to be re-homed. 
They can re-home dogs in Bulgaria, Germany, The Netherlands and the UK using reputable, licensed transporters and all dogs travel to other countries under the TRACES scheme as required by EU law.
Donations to support their work are always gratefully accepted.