I've been trying to find a material design CSS & JS framework that is able to provide a custom download of the components.
In other words, and for example with Material Design Lite, if I only need radio buttons, is there any way for me to download only the CSS and JS? What is the fastest way to get this custom CSS & JS files?
You need to download the main repository (either directly or via NPM) and then include just the radio JS and CSS in your build. How you go about this depends upon your existing build pipeline for your project.
MDL does not provide component-level resources directly since the need for that isn't great enough and with our color combinations it would lead to an exponential amount of extra files.