You can't use Mk III or IV until you hit Lt. Commander, so don't worry too much about it now. There are people on DS 9 that sell them for EC (and I think someone on some other station?), but the best way to get Mk IV gear is either through discovery badges or PvP medals (or both). But you won't get the right badges until you get the mission to explore the Hromi Cluster, and you won't get the right medals until you are a Lt. Commander.