I’m level 140 and have not seen any Mythic Unique weapons/armor… I haven’t really started Vessel of Hatred yet, would that be the reason or are they a seasonal item(s)?
The ladder bosses (Grigoire, Varshan, Beast in the ice, Lord Zir, Durial, Andariel) drop mythics. Higher chances with Duriel/Andariel. More chances/More loot on higher torment difficulties.
Undercity with the unique tribute has a good chance for mythics.
Tiny chance for mythics to drop anywhere. I’ve had 3 drop outside of bosses/Undercity.
You can also craft them if you have resplendent sparks. You’ll need the right runes too if you want to choose which Mythic you craft.
Good luck
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Ahhh, thanks for the info everyone. 
Do I need to be at least on Torment 2 for Mythic item drops? I looks that way.