Perhaps Lt. Graal will hand them out when you do explroation misisons? Or they are not being dropped currently. That begs the question though: how are Mk 3 weapons ending up on Exchange if nobody can buy them with exploration points? PvP?
I've already found some mk IV and V Commander level items from the level 14 and 15 missions. I think the Laurentian system fleet action is the best place for high level loot. You find tons of awesome crap there. It's a hard MFer without good people though.
I can't login to check right now of course but I think that's the one I got all my high level stuff from. Not 100% sure though.
Honestly I'm kind of disappointed in the level of quality between ranks. Mk III stuff is hardly much better thank Mk II it seems. Well for some items anyways. Torpedos or shields or ground weapons for example. Sometimes the difference is pretty big though like for impulse engines, they have a very noticable/tangible improvement.