Hi, my partner is Serbian and we have travelled to Serbia on many occasions by car.
We travel via Hungary and cross the border at Horgos, You will not be able to enter the country without purchasing Car Insurance for Serbia. At the border your passport will be held untill you produce your insurance document for Serbia.
You can purchase this insurance which is approx 150 Euros for one month, in one of the offices based on the border which is open 24 hours. You can pay in Euro's or change your money to Serbian Dinar at the border, again foreign exchange is open 24 hours a day.
We are also lookikng for an insurance company in the UK that will cover Serbia via a green card so if you come across a company that offers this service please let us know.
Happy traveling.
Thanks