The lady who redeems EDCs has a list that tells you what drops where. The only bit I can remember off hand is that the ground stuff drops in ground missions, space in space.
Once you get 2/3 of the XII for one set, you could spend the down time trying to get the same 2/3 for the other set I suppose. Not much of a consolation, I know. It would be nice to have some equivalent of the EDC for elite missions only, even if it was like 50 elite runs for each mk XII piece.