You can get the overall SmartBear plugins reference here: https://smartbear.com/plugins/
Locate the required plugin and click its brief description to get to the plugin details.
For example, the Git plugin details can be found here: https://github.com/SmartBear/ready-git-plugin
In the releases section, you can find the plugin .jar file. To install the plugin, you need to put this .jar into the '<UserFolder>\.soapui\plugins' folder on the host where Ready! API is used.
Also, here is a direct link to the Git Integration Plugin v. 1.0.2 (this reference is used by the Plugin Manager): http://productextensions.s3.amazonaws.com/SoapUI-Plugins/ready-api-git-plugin-1.0.2.jar
I hope this information helps.