devtrackers.gg
It shows up in the store now and then if I am not mistaken, just have to catch it.
We did sell the helmet, but not the armor set.
Edited: corrected myself.
Really? it will be in random rotation? cause I check every day in all consoles (ps4, switch, xbox) and doesn't appear hope you are not confusing it with the sentinel armor.
Perhaps you are correct; I was able to purchase the middleman helmet before, but not the armor.