It's nearly here!
The new Pokémon Go! Plus wearable is for sale Sept. 16th and though it's currently sold out on Amazon.com, you can keep an eye on it's availability here.
As for how it works,
At $35.99, the new wearable is cheaper than a new Apple watch and compatible with both Android and iOS devices. In the package, you should get both the device and an initial wrist strap, but you can also accessorize your Go Plus with Shield Skins and even a cool Power Bank to make your hunts longer.
Once it's released there should be a TON of third-party sites selling skins, alternative wrist straps, and accessories for it. From the looks of the pretty basic design, you can also probably pretty easily DIY your own wrist strap.
You can also just look super cool hanging it out of your shirt pocket--sans accessories--too:
So get out there and catch some Pokémon! Happy Hunting!