How Can I get sentiment analysis from a source or entity from iKnow REST API?

The SwaggerDoc for the iKnow REST APIs is also available online. Attribute information is relatively fine-grained and associated with entity occurrences aka sentence parts, which you can retrieve per sentence from the /domain/{domain}/sentences/{id}/details endpoint. The /domain/{domain}/entities series of endpoints are for unique entities, with which no occurrence-level information such as attributes is associated. Of course, you can also let attributes get rendered with highlighting rules, but I believe you want the attribute information in a more raw format, right? As an alternative to the REST API, you can get it from the SentenceAttribute and SourceAttribute views for your domain, as configured through the objectsPackage attribute in your domain definition (which you can of course wrap in a simple REST API of your own).

You might already have seen this article which, despite dating back two years, provides a full overview of how to feed and use sentiment attributes.