If you accidentally deleted or lost one of these items, you will need to obtain Tokens of Promise, available through many daily quests, as well as hourly quests at Hexe Sanctuary and Saunil Fortress (Becker at Hexe Sanctuary Analysis & Bacho Ladericcio at Suppress Shadow Shrine respectively).
Once you have the tokens, you can exchange 2 of them for any of the offhand weapons at the NPC Ronatz the Arms Dealer in the Capital of Calpheon.
In the case that it is the armor you need, you can simply talk to Grandus the Armor Dealer instead, standing just a few meters away from Ronatz.
Please note that any character that has already completed Magic Weapon Exchange II or Magic Armor Exchange II will not be able to exchange their tokens with these NPCs.
This means that if you have already completed the quest on your main character and are looking for Dim Magical items on an alternate character, then you need to trade the tokens with the NPC using that specific character and not your main.
With all that in mind, you should now be able to get any type of Dim Magical Armor or Weapon and be able to complete the rest of your quests!