Are you using SketchUp for commercial purposes, or for a personal hobby? If (and ONLY if) your use is personal you could download SketchUp Make 2017, which still works with most extensions. It is NOT licensed for commercial use.
It is no longer available from the official SketchUp download page, but you can search this forum for a legitimate alternative source, for use on either Windows or Mac operating systems.
I found this post with a link for Windows.
If however you are using SU for commercial purposes, your only option to use plugins or extensions is an annual subscription to SU Pro.