Recently I got into a discussion with my Longsword Fencing instructor about arms and armor around the Agincourt period, 1415.
The discussion went from the construction of bascinets to the use of Klappvisors by the "German" areas. But when I asked "What were they using in Hungary and the rest of the Balkans at this time?" We had a really hard time finding archeological or contemporary representation of these items.
If any of you know of a good book or source discussing South Eastern Europe during the Medieval Period that would be greatly appreciated.
The books of Dr. John V. A. Fine are a few resources that I've found discussing the Balkans around this time period. The second book might be closer to what you are looking for.
Dr. Florin Curta also studies that area and time period, but his work seems to be a bit earlier than you were looking for.
Dmitri Obolensky's "The Byzantine Commonwealth" is a good discussion of relations between Byzantium and the Balkans in the period, though it branches out to Russia and the Caucasus too.
I recently read Realm of the Black Mountain, which is a history of Montenegro by Elizabeth Roberts.
I know this isn't what you want to hear, but the biggest issue is that most of the sources you are looking for are in their native languages. All these countries have had a thriving industry, particularly in the last century, of histories focused on their own culture. We can pretty much pinpoint the collapse of Ottoman control in Southeastern Europe as a nice starting point to look for relevant texts.
My suggestion for finding information so particular is to reach out to museums and academics in these countries directly. There's a good chance you will find some people that speak English, and they can help direct you to relevant sources or even give you the TL:DR of the sources available in their native tongue but not English.
Im Serbian from Balkans and Im surprised people know so little about our midle ages, since we know a lot of our history by heart, we almost live in the past.