There's one that shows up under different sellers. But it's the same case. It was $40 but now up to $100. They seem to sell out very quickly. Just look under pixel battery case. Should be the first in the list.
Sorry, I read that wrong. I've only found regular pixel battery cases. I would hope by the time of the pixel 2 there will be more options.
There are practical limits to the thinness of Li-Ion batteries. And then, you have to surround the battery in a plastic case of some sort.
Honestly, I would rather carry my phone and, when I think I might need it, a separate battery capable of providing at least one full charge. Not too big nor too heavy, and easily kept in my backpack or a pant pocket (especially cargo pants ).