I don't know if they are really "missing" or just not suppose to be there, there are no phaser dual cannons for any of the other tiers. What's even worse is Tier 3 memory alpha has TWO energy ship weapons at mk X. Simply put there is no way to get mk x phaser cannons without drops right now. I managed to sell a Phaser Dual Heavy Cannons Mk IX common quality for 350,000 credits in <30 seconds of putting it up. Yeah it's pretty bad.
Hope for a random drop. That's about it.
Just like how there's no way to get EPS Flow Regulators short of MK VIII and X from any store.
I made the fatal mistake of not choosing the EPS Flow Regulator as a mission reward yesterday. I thought I can pick one up with my Exploration Badges. I was regretting my decision the next time I went back to Sol to look at the vendor. FFS. Of course the vendor had plenty of the item that I chose in multiple tiers.