Either using Googles Material icons site, Or using this git hub project that someone made that includes all of the different types of material icons that google doesn't include as web fonts, but as individual SVG images, like outlined, rounded etc.
It would be great to be able to use these on the Touch Bar.
Yep, they are already on my TODO list to integrate them into the picker 
You can already use them (like any image) - just download the png from the google material icons site and drop it into the icon view in BTT.