The sass files of the angular-material repository.
Installable via bower
bower install -S angular-material-sass-files
If something doesn't work, check if the versions of this package and angular-material (roughly) match. The current release of 1.1.3 is compatible with the current release of angular-material 1.1.3.
Please note that you have to manually adjust the theme files for the dynamically injected JavaScript values, if you want to use those. They can be found in the themes folder. The same is true for the checkbox mixin unfortunately.