There are several hotels near the Faculty (address: Bulevar kralja Aleksandra 67) that offer reasonably priced accommodation- the closest being:

Other hotels in the city center which are not far away from the Faculty (20min walking distance/7min by cab) are:

Last but not least, you may want to consider renting an apartment in the city center for a VERY reasonable price:

or CONTACT if you are interested in one of the following three apartments:
https://www.airbnb.com/rooms/2086364
https://www.airbnb.com/rooms/2308652
https://www.airbnb.com/rooms/2308874

[State that you are coming for the Belgrade Open Pre-moot at the Faculty of Law, a discounted prices may apply]

Many teams that have used this option in the past were very satisfied with the service.