I was trying to find one of those too but they are rare and when I found one, it was expensive. I broke down and bought a decent Corsair CX750M on sale and it seems alright but the system it's in is hardly low power. I have just built the X99 system I was waiting for and it's surprisingly efficient at an idle of 46w at the wall but that is miles from the range you are looking at. My X79 build never dipped below about 61w.
Just for a comparison, my x99 system has an E5-2650 V4 (12c 24 threads), 32gb ECC DDR4, 1tb NVME, 256gb SSD, P600 GPU and a 2.5gb NIC installed. There are mechanical too drives but they are spun down at idle.
My S12 has a 256gb NVME, 2tb SSD and an external 4tb USB drive and USB 2.5gb ethernet. It idles at 7w, peaks at 30w.