My wife and I will be spending 8 days in northern Croatia in August/September and intend to hire a car in Zagreb on day 1 and drop it off in either Porec, Rovinj or Pula – depending on which town has the ferry leaving for Venice (our next destination). So far I have been unable to confirm ferry timetables (we would be leaving Istria on a Thursday).
I have also had trouble finding a car hire company where we can pick the car up in Zagreb and drop it off in any of these three towns.
We will be doing the same trip in August/September this year. Can’t you rent a car through Budget Rentals or Hertz Rentals at the Zagreb Airport. You should be able to book that online. Good luck I hope it works for you. Let me know how your trip is.