There are NPC shops that sell Armor in all of the realms, however as there are potentially at least 20 different items of armor available each realm it is not possible to have a shop for each available armor piece.
So there are generally only two NPC shops that sell armor per realm.
There is the Level 5 Armor vendors inside Springlade in the first realm, then level 30 sellers inside Northglen, and level 50 sellers inside Coldglen.
Outside of those levels you will either need to scavenge for new armor by killing humanoid monsters, or by crafting armor using the relevant crafting skills.
You can also buy armor from other players, check the marketeer to see who is selling what.
If you do not have any crafting skills yourself but can get resources you may be able to use the Craft marketeer to get what you want made.
Another thing to look for are Dungeon armor chests, these drop armor which can be adjusted to suit almost every level (5 level increments), you need to earn prize tokens inside player dungeons, and redeem these at a dungeon prize chest to get a reward.