Treibh Caillte dungeon, definitely drops there. Can't remember exactly what now (from memory a few TC mobs do - not Thralls though, they drop a different reinforced armour - may have that back to front though), but I THINK some dropped from Ursine Patrols. Pretty sure a lot of this reinforced dropped from pulling the King's room too. Definitely not an uncommon drop there.
Sorry been ages since I have been in TC, and I was hunting scale armour at the time.