Information about getting insurance in Bosnia seems really sketchy, this thread aside, and it was the one I was most concerned about on a recent trip. It was also the only country on our route that I couldn't get a green card for before leaving. So, my experiences from that trip, in case they're of any use.
Serbia: Entered near Dimitrovgrad from Bulgaria. No paperwork checks whatsoever, just a cursory flash of the passport and a disappointingly faint stamp. Serbia's now signed up to the agreement on motor insurance so EU policies should be valid.
Montenegro: Entered south of Novi Pazar, can't find the name of the border crossing itself (I told our local contact we were entering at Granicni Prelaz, and then felt like a right plum when he told me what that meant). They wanted to see full paperwork on entry, though like everyone else on the trip they were a bit baffled by my white green card. Didn't notice anywhere there to buy one if I hadn't already got cover, and they really were keen on the paperwork so probably not a great place to rock up in hope of buying at the border.
Bosnia: Entered from Montenegro at Vilusi, just east of Niksic. The chap in the booth only knew two words of English: "green card". Alas, I didn't know enough local lingo to explain that I didn't have one and needed to buy one. Eventually another chap appeared and spoke enough English for us to get things sorted.
My passport was stamped, and then I had to park up just after the booth and walk down to the second building on the right where another chap behind a desk was happy to sell me 3 days' cover for a bike for €21. Once I'd got the insurance cert I just had to walk back, show it to the chap in the booth, and I was free to go.
Croatia joined the EU while we were away, and entering from Bosnia at Gradishka was just a simple paperwork and passport check. They wanted to see vehicle documents, but didn't know one side of a DVLA V5 from the other so it was a bit of a waste of time. UK policy was fine though, it being the EU.
The only country that was proper ball-ache to enter was Turkey, and even that was easier than I expected.
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