First of all, here is the link which is rather useful for Croatia bus rides -
http://www.autobusni-kolodvor.com/en/timetable.aspxBus rides isn't that cheap in Croatia, so traveling by bus to Zagreb would be more expensive then by train, where you get 25% with ISIC. On the contrary - it's not that easy to get to the closest train station.
With Zagreb-Belgrade everything is relatively easy - regular train ticket Zagreb-Belgrade cost app. 30 euro, but you can save some money (like 8-10 euro) if you travel on a domestic HZ ticket till last Croatia station Tovarnik, Tovarnik-Sid with international ticket (or even without it), then domestic ticket Sid-Beograd.
There are also buses from Zadar to Belgrade, takes a little bit more than 10 hours, fare around 300 kuna