I'm a newbie in Alfresco development, just want to ask as to where can I find the org.alfresco.web.action.ActionEvaluator to create a custom evaluator class or if there is an existing jar file regarding this?
You can find this class in - alfresco-web-client-your.version.jar which is existing jar.
To develop custom evaluator, You can extend this class BaseActionEvaluator which implements ActionEvaluator but this is for Alfresco explorer NOT for Share actions.
As you have posted in Share development, I am assuming you need to have custom evaluator for Share
To develop custom evalutor for Share action you can check this class - org.alfresco.web.evaluator.BaseEvaluator[alfresco-share-your.version.jar] and some existing example.