Xbox One X Pre-Orders Start TODAY! (~12pm PDT / 3pm EDT) MSRP $499.99
UPDATE: Pre-orders are no longer available at any of the retailers below, but they've been popping in and out of stock on occasion, so keep checking the links below if you're still looking for one. You might have luck if you try to pre-order in person at a Microsoft store.
Here are current links to retailers' Xbox One X - Project Scorpio Edition pages:
8/20: Microsoft's Gamescom press conference is scheduled for today at 12pm PDT / 3pm EDT. Pre-orders will go live at some point during or following the press conference. You can watch a stream of the conference on Mixer's Xbox channel - https://mixer.com/Xbox.