I want to put ads in my game built using Godot 3.1, but I can't find the plugin and source for Google AdMob.
How can I incorporate ads?
The Android plugin system has been through some changes. Some community maintained plugins have not being keeping up, and others did not bother to back port to earlier versions. So you need to pay attention to which version of Godot the plugin supports, and follow their instructions to set it up.
In fact, for Godot 3.1 you would likely find Android plugins in the form of modules. Which require to compile custom export templates for Android. You need to follow the documentation for your version of Godot (for Godot 3.1 it is this one: compiling).
I want to point you to a similar question I answered over StackOverflow: GODOT- how to deal with godot source file, custom modules , and how to import projects?.
For what I can find, this module supports Godot 3.1: kloder-games / godot-admob . Barring that, try the one it is forked from: Mavhod / GodotAdmob.
You might run into other kind of compatibility problem, because these are modules, and you need the correct SDK to build them. Plus, you probably want to keep up with newer Android SDKs (Why target newer SDKs?).
I believe the change in the Android plugin system is for the better, and that the risk of upgrading your project is marginal. So, I recommend to try to upgrade your project (and again I remind you to make sure you have a backup, just in case).