USB3.0 Internal Header Connectors
Yep, this is a tough question. The body of plug (wire side) has 5.2 mm width, and length of 20.65 mm. None of Molex-Amphenol-etc products match it. 

The connector seems to be solely manufactured by Foxconn, an old Intel ally and platform manufacturing house. There are some re-sellers like ModDIY.

Or see this e-Bay listing if you need few parts.