Saro, the dwarven blacksmith, can be found in the south-west corner of Keldagrim (south of the Black Guard H.Q.). This building can be located on the map easily by looking for both the anvil and shield symbol within it. He sells multiple types of armour in his Quality Armour Shop, ranging from steel through adamant chainbodies, steel through adamant medium helmets, not including black (which can not be bought at any store, other than the Grand Exchange), Steel sq shield, and black kiteshield. This is indeed the only store where black kiteshields can be sold in RuneScape.
There are multiple ways of getting to Rellekka to access this store now: the tunnel entrance east of Rellekka, through the trap door in the north-east corner of the Grand Exchange, Dwarven Mine, Dorgesh-Kaan, or underneath White Wolf Mountain.
What makes this store truly unique is that it sells the rare and hard-to-find black kiteshield, which else where can only be found from Ice giants and Spiritual warriors. However, it is only stocked by players; meaning the store does not have an inventory of black kiteshields (remember that black armour can not be smithed from raw materials, only found from drops or bought).