Add eye tracking to your HTC Vive headset for $149 | Engadget
On Sale Details: Cosmos Elite Headset (HMD Only) and External Tracking Faceplate, both with Half-Life: Alyx | VIVE Blog
Amazon.com: HTC Vive Cosmos PC Based VR Headset System - Bundle with 2 Pack Vive Tracker for VR Headset, Rebuff Reality TrackBelt + 2 TrackStraps Full Body Tracking VR Bundle, Black : Video Games
HTC Vive Cosmos and Cosmos Elite review | TechRadar
Vive Cosmos Rated Least Accurate Among Top Headsets in Controller Tracking Test – Road to VR