In Belgrade there is an Antiquities fair every month in hotel Slavija (try finding info online for "Sajam antikviteta Beograd", one in Feb was held already). It can be really good. Street/antique shop vendors are usually not that great and have mainly thrash with every worthy thing heavily overpriced. Same in Sarajevo, where you will find them in the center, but probably do not have anything interesting. There might be some flea markets on the weekends on places where fruits and vegetables are sold on the other days, keep an eye for these.
