How do I improve the AC of the monk without mulitclass except via Dex/Wis? 

Which magic items or Feat are there still?

I have a level 15 Way of the Ascendant Dragon Monk that has Dex 18 / Wis 20.

These are the elements that enhance AC that come to mind at the moment, there are probably others.

Objects:
Bracers of Defense, Cloak Of Displacement, Ring Of Protection,

Feats:
Defensive Duelist, Magic Initiate (Shield, Shield of Faith,..), Fighting Initiate (Mariner (UA))

also if your allowed any race you can choose simic hybrid and choose the carapace animal enhancement at 5th level for plus 1 ac.

With a kind GM maybe Staff of Defense, though you could not use the spells in it but +1AC is from it is good.

Objects:
Bracers of Defense, Cloak Of Displacement, Ring Of Protection, Staff of Defense,

Feats:
Defensive Duelist, Magic Initiate (Shield, Shield of Faith,..), Fighting Initiate (Mariner (UA))

Thx 

sorry I missed the part that you had already created your character.