I guess persons you talked to are not well informed. Cadastar maps of Serbia are available only if you pay, and cost is about 500 EUR per sheet.
I agree that such maps should be freely available as they are all created using our tax money, but it is not the case. Using them without proper approval is illegal. 
cirko, do you have legal rights to upload all those orto-photo images at Geothings MapWarper? Maps you placed there are from RGZ and it they surely cannot be used in manner you do. And you did not georeference them properly! Why are you doing this? It is illegal, it is not allowed by OSM and you produce false data into map. No one can benefit from that!