We used to have a lot of bands during the 90's (check out Hard Core Serbia 90's... people regularly post links to albums and videos, if you're interested), but in the past few years it seems people have lost interest in hc. I'm still hoping it will pick up, and a new wave of people will come :)
As far as bands today go, there aren't that many. I see already you have added Let's Grow, Unison and Jaibo! (all great bands, kudos :))
Check out Ha-Ko Bastards (all the bands listed there, are from Serbia, except where noted. You can download a few songs from there too, so give it a listen :)
Path Of Decay recently released an album, but it isn't listed there yet, so definitely check them out.