Hmm, with this closed I'm not sure.
Have a look at:
www.belgradeeye.com
Under sightseing, museums are listed.
If you want to buy reproductions of serbian masters and fresh local art, there are several shops in Old Belgrade. There is shop with all things Serbian on Knex Mijhilova, I'm not sure of it's name but it has local art, precious stones, local drinks, a kind of art & craft shop. Walking from Terazija to the fort it's 1/2 way down on R/hand side.
You'll come across small craft shops and religeous shops that have icons etc. for sale, some of which is traditional local art reproduction. Also the shop by the church on Kalemegden. Lots of national artists paint chgurches, monastories and icons/ saints.
Stoofer