It was difficult....but we found it !! It is located at #75 Kralja Petra I, next to a photo shop, across from where the Ferhadija Mosque used to be. The woman working there was very nice and extremely helpful with information about Banja Luka, and BiH / Republika Srpska in general. We were able to get a prehistoric book of pictures, which also included a city map showing all the mosques (which were destroyed in the war).
The bottle (0.33l) of the "HEINEKEN", in coffee bars in Gospodska Street, coasts, 3.5 to 4 Convertible Mark (1.95 KM = 1 Euro).
Banja Luka has many hiden natural gems even for locals.
Regardless of whether your choice is a tour excursion around the city (Banj brdo) or visit another city and its attractions...