MDBootstrap.com is the best website around if you want to use Googles Material Design. The whole content of this site is focused around connecting Material Design and Bootstrap. Their themes are built with the latest version of Bootstrap and you can use them for free, no matter if for personal or commercial use cases.
The site is extensively rich of content - if you have any questions around how to correctly use their special Bootstrap Material Design Framework you will find the answer to that on their documentation sites.
They are not only offering some free themes for Bootstrap, instead there are many aspects of web development integrated into their site regarding the development with Bootstrap and e.g. javascript frameworks like Angular, React and Vue, and also other libraries like JQuery. On top of that there are also many tutorials and resources on how to efficiently build your website using their tools.
It is somehow crazy how big this system is, but there are many people behind it actively developing it further.
