This is actually a bigger problem, maybe you can get it to work by using archive.org, though? Maybe even as repository?
The problem is you need to checkout the jmonkeyengine repo for that (on 3.0), go into the sdk folder and then somehow grab the plugins which were splitted as part of the jMonkeyEngine-Contributions github user and then you need to somehow copy them into the sdk folder into the right place and build them from there.
I was trying to move on to gradle and 3.1 and it works partially but that’s another story.
For other people who need other plugins: You can add plugins from local store with Tools->Plugins->Downloaded->Add Plugins. Maybe you can compile the plugins found here: jMonkeyEngine Contributions · GitHub