Here's a website that I'm referring whenever I'm about to check exchange rates for different banks (although I'm looking for average exchange rate, not for certain bank) enter link description here
As far as I know, they're using National bank of Serbia exchange rate as a reference enter link description here
Certain banks such as Credi-Agricole and Societe Generale have a "Non-guaranteed exchange rate" which gets better conversion rates when chosen.
I put my Mastercard, choose that option and withdraw certain amount of Serbian dinars (RSD)