Looking around, I see the libraries here: https://www.b4x.com/android/documentation.html
But when clicking on a link to investigate the libarary, I cannot find the zip file to download?
The link you posted is for the documentation, to find the download search the forum for the library name. The download will be in the forum thread, usually attached to the first post. Unless it's an internal library, in which case you don't need to download anything, it should already appear in the Libraries Manager Tab in the IDE.