I doubt they have nuclear reactors (Bulgaria has one, or had anyway), so any material (uranium in relevant form) is going to be pretty scarce in the first place.
Now if Isis somehow has the tech to build a nuclear weapon, this is not really down to homegrown supposed caliphate-rebirth groups, and it would be given by current nuclear powers. And in such a case (i seriously doubt it is true) that power is not likely to have been Russia. More likely some USA groups, or Nato at any rate.
Of course it also depends on the scope, and what one means by 'dirty bomb'. If one means just any kind of somewhat not utterly failed nuclear bomb, then maybe something can be built, but they will have not tested it anywhere so it is a very unlikely scenario to happen. Now if you fear a north-Korea type situation where Isis is backed by a nuke, well, be sure this won't happen if a superpower doesn't actively enable this..
(of course, if rumours about Israel having a nuke or a few are true, there already is a very self-righeous and violent polity that is shielded in such a manner. Personally i don't buy the whole 'Isis are self-grown islamic warriors who are winning and now have a state' tale. It is virtually certain they are being backed by actual relevant armies on the ground).
