In the last episode of Durarara!! we see Izaya with Celty's head. How did he get the head?
Izaya traded Celty's head from Namie Yagiri for a favor. Namie works in Yagiri Pharmaceuticals where Celty's head was stored at that time. Namie had access to the storage room and didn't need the head that much, so she gave it to Izaya.
Also see this excerpt from a Durarara wiki:
At his apartment, Izaya meets with Namie once again. He comments on her situation, giving up all she had for her brother's love and having her company being bought out by Nebula. He agrees to help keep her in hiding in exchange for Celty's head.