If you want to buy the remote separately for the Apple TV device you already own, you can get it for $59 from Apple. I can place an order for delivery on Thursday or pick it up on Friday.
The design of the third-generation Siri Remote is the same as that of the last generation, which was a huge improvement. The new remote is compatible with all generations of the Apple TV, according to Apple.
Over the past few years, Apple has begun to switch its products away from lightning charging, with the iPad line just completing the switch today. The decade old charging port is still used by many Apple accessories and devices. It seems likely that the first product to make the change will be the Siri Remote.