To find the Servlet.jar on your system you need to download a Servlet container or a J2EE SDK Server.
You can download either of the following
WebLogic or any other Server
It is shipped as a part of the server and is not meant to be downloaded separately. If you are using Tomcat, you can find “Servlet.jar” in your tomcat installation folder (tomcat\common\lib)
You would either find the servlet.jar or servlet-api.jar
You can get latest Tomcat Version here;