seat61.com is he site with best current info on how to bridge gap for dedicated Iwantitallonrails people. Certainly Sof-Bgr was recently updated.

Just use google for buslinks between towns-there seem to be 2 or 3 mostly aimed at old Yugoslavija, but be aware that there simply is very little to no demand for such short hops on bus or transit- IF you are so stubborn to want it as short as poss, then it often means bus to last village pre border-walk/taxi over, and repeat process reverse. Or google :: busstation timings in X. Every now and then some hard-core railfans -who always seem to have too much coins- boast about ´taxis are very cheap there´ which as such in comparison is true. You could even try such apps as bolt or uber to check what turns up.

DO be aware that train timings are not set at all to make such trips a smooth affaire-and RS=Serbije is also noted for many last min. cancellations of trains: train broke! That is simply how it is.

Really best place to ask for /look would be BAS= Beogradski Avtobusni St, which still sits on prime space beside the now torn down and left to overgrow with weeds old main station.