My friend plays chaos space marines with the old codex. We want to play maelstrom of war and I think that having datacards to draw objectives is very handy. I can't find a chaos space marines datacard deck anywhere. What deck should he use? There are chaos daemons and chaos daemonkin, but I reckon that these are different armies from the chaos space marines. Is there any other option?
You have two options - there is a basic set of objective cards that is not army specific - any army can use these. Additionally, the book "Traitor's Hate" details a new, Chaos Space Marine specific set of objectives that can be used.
It does not appear that you can buy these as a set of cards on their own, but it would not surprise me if this changed eventually.
Additionally, the only difference between any army's specific objectives and the generic objectives are in objectives 11,12,13,14,15,16. You could use the basic cards, and replace those when they appear with the ones in the book (though, this requires you to pay more attention when drawing).