Lonely Planet mentions that there are buses going from Zadar and from Rijeka to Trieste (or Venice) but doesn't mention a company or a web site.
Do you know which companies and the web site?
We will be traveling in Croatia and need to catch our flight back in Venice at the end of June 2013
You can check the bus schedule from Rijeka to Trieste here:
i am afraid there are no lines to Venice from either Rijeka or Zadar so you will have to change busses. There is a boat line to Venice from Pula and Rovinj that you may want to consider. about 6 years ago