The base version of Nilfgaardian armor is available for sale at any character level lower than 35. However, it can only be equipped starting at level 10. At level 35, this armor is replaced in the quartermaster's inventory by its New Game + variant.
New Game +
A higher-level variant of the armor is added by New Game +. It can be bought in the same way from the Crow's Perch quartermaster, but only if the character level is 35 or greater. Despite this being a New Game + variant, it's also available in normal game saves if your character's level is high enough.