Best answer: There is only one place to buy the Valve Index and that's at the Steam Store. There are a lot of bundles to choose from, but they all have to be bought through Steams own store.
The new reference for Valve VR
When Valve decided to make its own VR hardware they really swung for the fences. The Valve Index is a supremely high-end headset with a plethora of features that should help it be a cut above the rest.
The Index has custom made lenses to help focus on the dual 1440x1600 RGB LCDs, and those LCDs can run at a staggering 144MHZ. All the work they have done is to reduce Screen-Door Effect, increase your Field of View, and make the screen as immersive as possible.
Added to that Valve has created a new off-ear headphone system it says will help aid in immersion and allow you to play for longer periods of time. You can tell they have spent a lot of time making you feel good on the inside.
Not only have they worked on what you feel on the inside of the headset, but they have also added cool features such as an expandable USB 3 type-A socket on the front. This will allow third-party developers to create cool peripherals that we may never have seen before. When you add the cameras to enable video pass-through, the Index is shaping up to be a powerhouse in VR gadgetry.
Where can we preorder the Index from?
The Index is Valves crowning achievement and as such, they are keeping the preorder close to home. At the moment you will only be able to preorder the Index from the Steam Store itself. No other preorder locations have been made available. Perhaps after the release, they may allow other vendors to sell it, but right now it's the Steam Store or bust.
Happily, you can preorder the Index in parts. If you already own a Rift or Vive, you may already own the Base Stations and controllers. In that case, you only need to pick up the headset on its own. Valve has made all the components backward compatible with those accessories and all of them are available individually.