I found this in the Internet:

After some posts on the subject (and a rather uninspiring discussion with a fellow poster) I discovered a possible solution for motorists from the UK who need compulsory insurance Green Cards for countries like Albania, Serbia and Montenegro, Belarus, Bosnia and Herzegovina, Macedonia, Moldova, Ukraine, Croatia etc. as well as Israel, Tunisia and Morocco.

Usually, UK insurers refuse to issue these for countries other than EU member states, which makes it necessary for motorists to buy separate and expensive insurance at the borders, especially for Serbia and Montenegro.

NFU Mutual seems to cover these countries, see p. 14 of this leaftlet (it's p. 16 of the .pdf document):

www.nfumutual.co.uk/policy-wording/documents/MT010...f

It doesn't state clearly that a Green Card is issued, but I suppose that it isn't a problem because otherwise, that cover would be pointless as a Green Card has to be presented as proof of insurance at most borders of these countries.

Perhaps someone is willing to check. Perhaps there are also other UK insurers that provide the same cover as NFU Mutual does only seem to insure rural drivers.

This guy seems to have been wrong:

To enter Montenegro with a car you have to have insurance. Nobody in the UK sells insurance for Montenegro but they have this covered at the border and it all works, if in a happily chaotic way. You drive up to the first border police barrier and he asks for your passport, V5 registration document and Green Card for insurance (but not driving licence I note!). You tell him you don't have the insurance yet and need to buy it, so he keeps your passport, tells you to drive through the customs point, park, go and buy it, then bring him the certificate. Dead easy. I drove past the customs man who waved me through without slowing down, parked and walked over the road to a building, gave a random chap who may or may not have been just hanging around outside it 15 Euros and he gave me a bit of hand-written paper. This is where it then departs from the expected slick operation as I then just walked back up the middle of the six lane road, straight through customs again along the road and up to the Border Police barrier. He handed me my passport back then I walked back down to the customs gate, where the chap lowered the barrier and said "anything to declare?". He'd let me drive through without a word, then stopped me when I was walking through! He looked so forlorn in his little booth that I very apologetically said "no, sorry". He shrugged, looked down and said sadly "they never do".