??? Is there a central bus terminal in Zagreb where we can book tickets for all of our journeys in Croatia ???
Yes, there is a Central Bus Terminal in Zagreb – it is called Autobusni Kolodvor Zagreb – this is their home page : http://www.akz.hr/
You can book a lot of journeys from there, but not ALL bus journeys in Croatia.
Buses are very frequent around , and you don’t need to worry about booking your journeys well in advance.
Day or two in advance should be fine.