The folks behind Lord of the Rings Online very recently announced that in Book 13 there will be new vendors that will allow you to purchase the armor sets that was announced to come up with Book 12 -- unfortunately not all of the sets were made available. Book 13 will attempt to fix this though!
When the Great Angmar Revamp came out in Book 12, a number of armour sets were added to quests in the region. As sometimes happens, though, we were a little overzealous with the plan for new armour sets, and they ended up on many quests that were already in-game in some form prior to Book 12.
This meant that it was possible to complete these quests before Book 12 came out, and if you did, you would never be able to finish some of these new armour sets. Few things can frustrate so well as uncompleteable armour sets, judging from my PM inbox (love ya, everybody!), so we started looking for a solution.
The one we've settled on, and which will be coming in Book 13, are a number of vendors who will carry each of the pieces of these armour sets. You'll be able to purchase individual pieces of these sets, and if you've completed the quest that rewarded them, you'll be able to equip them as usual. These won't be cheap, as in many cases these would be a double-dip on quest rewards, but if you've been carrying two pieces out of a three piece set, this should let you complete your collection with a minimum of hassle.
Vendors for these armour sets will be in the Búth Sánkhas and Búth Luikh in Aughaire, and in Gabilshathur.