Have you tried checking if a local arboretum or botanical garden has some? If they do, you could explain the situation and ask if it would be possible to take some cuttings or, assuming they fruit, fresh seeds. (Or just grab some, if you can do it neatly without damaging the plants or disturbing anyone...)
I don't know if this exists anywhere else, but in Belgium (my partner is from there) there's a search tool for browsing the collections of a bunch of arboreta and botanical gardens (
http://www.plantcol.be/search-plants.php). Unfortunately it doesn't cover all arboreta, but it's very useful. A search for Ilex vomitoria shows that one of the collections has 10 accessions, and another has 1. So it does exist within Europe at least. You could try to find a tool like this for Bulgaria, or if there isn't one check each individual arboretum. They tend to keep at least a basic list of what they have somewhere on their website, although actually finding the plants when you visit can sometimes be challenging...