Best research assistance/service you can get for your ancestors is locating descendants of the same family.
I have seen the reports *respectable* researchers provided for ethnic Serbian ancestry in neighboring countries, and I would definitely not recommend any of those that are advertizing their *professional* services and *expert* knowledge.
http://www.rodoslovlje.com/en/documentation/guide-researchin...Serbian Genealogy Society SGS "Rodoslovlje"
http://www.rodoslovlje.com/en