I would email the US Embassy in Belgrade. I'm sure the question has come up before.
If your trip is less than a month or so, I'd advise taking the supplies you need, plus a cushion to cover you in the event of delays. Whether vials or pens, insulin is light and small, easy to travel with. The only issue might be refrigerating an opened vial, though I personally have never stressed about that for a trip of less than a month. If you are concerned, every pensione, B&B, guesthouse or hotel I've stayed in has had a fridge somewhere.
If you can't possibly carry the amount of insulin you'll need with you, I'd go armed with a letter from your physician stating you have diabetes and require insulin injections or are on insulin pump therapy (the sort of generic doctor's travel letter you should have anyway, for customs and security checks). My understanding is that insulin vials and pen cartridges are color-coded to type, so even if a pharmacist can't read your Humalog label, you'll get the right thing because of that maroon cap.
Recent reports are that diabetics in Bosnia can't always get the supplies they need locally, so if you think you'll need to buy insulin, you'll probably have better luck in a major city. The US Embassy in Bosnia website has a list of hospitals. Worst-case scenario, you can always take a ferry to Italy for the day. In the EU, you do not (or should not) require a prescription to buy insulin, if you can reasonably prove you need it, such as by having insulin with your name on the prescription label, or unveiling your insulin pump.