Following their unveilings on Wednesday, Apple has made both the iPhone 6s and 6s Plus available for online pre-order. As usual, folks initially had a hard time getting orders to through on Apple's website, and I had a terrible time with AT&T, but all portals should be working now.
If you're still having issues, we highly recommend trying out the Apple Store app, which you can download here. It offers a more streamlined buying experience and supports Apple Pay. For those trying to pre-order an unlocked iPhone, you may want to check out our tutorial here.