Dear Karolinka, sorry for interfering, but I am just trying to be helpful about your new passion for organic boza:-) Well, you could find Harmonica brands occasionally in most of the hypermarkets in Sofia, but most certainly you could find it in the Biomag-stores! Why? Because the guys who own the production company of Harmonica brands are also the owners of Biomag-shops - there are actually three of them in Sofia as far as I know, but you could easily check and find out the shop with the most convenient location for you at their web-site here: http://biomag.bg/
Good luck and have a nice drink:-)