It's possible to find a rather good set of armour relatively early in The Witcher 3, the Griffin Armour (Level 8) set, as Redditor Mystic87 has pointed out.
Located in the Monster Den in the Velen – No Man's Land area of the map, the armour is guarded by a Ekimmaras monster, which is weak to the Igni sigil.
There are numerous wraiths in the cave, so be sure you don't confuse the two when fighting. The Monster Den is also a good place to farm dusts as the wraiths drop them here.
It's possible to get the armour at a low level (four or so), by waiting for the Ekimmaras to pass and running up to the chest and looting it before drawing your sword. Once you've done that, you'll need to get out of there pretty quick.