At this moment there aren't many CB's who will be achievable for us. Three names who could add quality:
Adil Rami, Valencia: This guy is a beast, watched him several games and he never disappointed. 25 years, starting spot in France NT. Great with the head, good feet. Probably the "easiest" to get
from the 3.
Mamadou Sakho, PSG: Only 21 years old but already captain at PSG, and several starting spots at the France NT. Left footed, could be a problem if Chiellini will be played back as a CB. But he has already great quality, and could easily play 10 seasons. But as a youth product from PSG, and the money there, very difficult to get.
Mats Hummels, B.Dortmund: He have a great pass, especially his long shots are amazing good for a CB. Also not that slow, and only 22 years old. A high price tag, but I think he'll be worth every penny. Like him.
I would pick Hummels as first choice, Rami second.