Where can I find JavaEE packages' sources?
Java EE is an abstract API. It exist of just contracts (as you see in javadocs), not concrete code. The application servers are the concrete implementations. So, if you're looking for the source, you should look at the application server vendor's homepage for a source code download link. However, you're unlucky with Weblogic. It is not open source. I've never used Weblogic (I am an open source fan), so I'm not sure if the source is provided along the Weblogic license, you might want to contact Weblogic support team.
Other servers, like Tomcat, Payara, WildFly, etc are open source. WebSphere has also a "Community Edition" which is open source. You could grab the javax.* API source code from any of them, but there is no guarantee that they are exactly the same as Weblogic uses. And still then, they do not provide the concrete Weblogic implementation code (like as the code in org.apache.*, com.sun.* and org.wildfly.* packages in aforementioned open source servers).
See also:
- What exactly is Java EE?
Try in the default Maven repository; search for "javax.ejb", then download the source. That's quite often the easiest way, saving you from clicking through EULAs and the like.
That said, this source code is only good for plugging into an IDE to get source code completion — implementations are application-server specific (see Balusc answer).