Unique items are placed in certain location like Kal says and some of them are sold by NPC. The problem is that unique items are only good for 1 or 2 items above their levels so you will still have to change your gear alot, and even more often if you find a good purple gear sold by NPCs that have the perfect stat because the sell gears that is equal to your level. (So you can end up meeting that level 4 axe merchant with him selling level 5 gears)
Certain purple gears and green gears are also placed as well and some of them are almost always dropped by some bosses you kill. Random type can also be found in some of the chest in hidden area with random item type but they are always useful anyway because they are found while you still barely have anything to wear.
You can't save scum the chest though because after they are discovered, seems like the seeds are saved unless you go through some long hoop that involves more than just quitting game and reloading.
You can save scum quests reward though. They are guaranteed to be a certain rarity so you can just reload until you get the right gear with the right stat to choose.
