Apple says the iPhone 8 and iPhone 8 Plus, and presumably the iPhone X, can be fast charged with any comparable third-party USB-C power adapter that supports the USB Power Delivery specification.
An official Lightning to USB-C cable is still needed, and it's not included with any iPhone. Apple charges $25 for a one meter cable, and $35 for a two meter cable, in the United States. There aren't any MFi-certified alternatives yet.
Apple's own USB-C power adapters that support USB-C Power Delivery include:
Aukey has a few cheaper third-party options available on Amazon:
Fast charging enables the iPhone 8, iPhone 8 Plus, and iPhone X to charge to 50 percent battery life in just 30 minutes.
Disclaimer: This article is not affiliated with Aukey, but MacRumors is an affiliate partner with Amazon and may get paid if you click one of the above links and make a purchase.