Local companies doing construction in Serbia will happily provide you with a plan as well as a building cost ahead of time.
However, plots and legal requirements differ significantly that in urban environments, it's hard to make the most of the plot without going custom (requirement to be that far away from the neighbour and the street, whether you can have windows facing that side or not, what kind of foundation is required by the terrain, etc).
You're probably right. It makes more sense to sell those mass-produced floorplans to architects, who will make slight tweaks for local conditions and perhaps pay via subscription, rather than directly to consumers who will probably just copy the floorplan .jpg and take that to the builder anyway.
However, plots and legal requirements differ significantly that in urban environments, it's hard to make the most of the plot without going custom (requirement to be that far away from the neighbour and the street, whether you can have windows facing that side or not, what kind of foundation is required by the terrain, etc).