Whenever I have to add certain library from the internet to my Android project, I add them inside the dependencies in the app level gradle script and it downloads the librar
Just go to Maven central and download the libraries.
For example, here is Volley. Just click the download JAR button.
I would strongly recommend sticking with Gradle / Maven, though, to keep consistency with versions and appropriately handle additional dependencies for the libraries you want to download. They are called package managers for a reason, and they do their job well.
The libraries are actually downloaded to disk only once and shared between projects, they aren't downloaded for every new project.