Finding train and bustime tables in this area is always quite hard :)

For trains i can say there is currenty no train usually there is a connection from Belgrade via Nis to Sofia but currently not in operation :/

Buses it´s the best to use the local bus station or you use a priate van transfer 

Geatours is just the famoust one as it have a website online and offer van transfer to Budapest or Sofia for 30€ from Belgrade