Should anyone be interested in making yogurt with Bulgarian cultures, they are available on ebay:
US: https://www.ebay.com/itm/Bulgarian-Starter-Kultur-fur-Tradit...
EU: https://www.ebay.de/itm/Bulgarische-Joghurt-Starterkultur-na...
I make Kefir myself with a culture that I once acquired on ebay. May try to make Bulgarian yogurt one day.