I think people are recommending the 1300W EVGA, which I believe can handle 3 Neptune modules.

I use 3x Corsair AX1200i for my Neptune, which I'm planning to expand to 5 of them for two Neptunes (when I get my bonus Neppy), i.e. one AX1200i powers two Neptune modules.

There is a catch with the AX1200i.  You need to have PCIe 2-to-1 splitters to use them with the Neptune because the AX1200i has Over-Current Protection (OCP) which kicks in and shuts the PSU down. I blame this on KnC, not the PSU, as the Neptune draws more power across a single PCIe connector than the specification is designed for (that's why the safety features of the AX1200i kick in). Putting the PCIe 2-to-1 splitter cables on means that two PCIe ports on the PSU feed the single PCIe port on the Neptune module, so each PSU PCIe port is supplying power within the PCI specification (and doesn't trip the OCP).

I got my PCIe 2-to-1 cables from Cablez, and I'm very happy with the product (see https://bitcointalk.org/index.php?topic=170332.msg7579936#msg7579936). I've been running them 24x7 since July 11th with no trouble (they don't get warm).