Other than random drops or loot from chests, once you get the battle tower there are a couple merchants you can check each time you level for better equipment (save before running your crosshairs over them, and you can reload until you get something you can use).
There are two black ring soldiers in the bunker where you need to get to to turn off the anti-dragon barriers; they will surrender, and one of them has weapons, armour and sometimes jewelry for sale. After that you can continue up the path past the depleted ore mine; if you keep to the left path when it splits, you go up to High Hall, there there is another merchant after you do a quest there. Taking the right path leads down to the Champon Harbor; there is a smith inside the building, and if you bring at least one of each of the 3 different zeppelin parts (found in wyvern nests) to the zeppelin master, you can travel to Aleroth, where there are 2 merchants who sell legendary quality items (as does the one in High Hall, at high character levels).
