Rental car companies.

You can check www.AutoEurope.com which brokers car rentals for all the big agencies— often at airport locations but also in city centers. Most car rental agencies will charge a hefty one-way dropoff charge — often $1,000– for picking the car up in one country and dropping it off in another. You’ll see this when you dry run a reservation. Because of the one-way dropoff fee, you might consider renting one car in Slovenia and returning it before going to Zagreb ( 2 hours from Ljubljana by train) and then picking up a separate car for use in Croatia.

We always use AutoEurooe for rental cars in Europe.
We have used TravelGuard and have had very good experiences with them when we had to make claims.