I assume that you mean org.jboss.jms.server.ServerPeer. This is part of JBoss Messaging. You can find the sources in the JBoss AS source download at jboss-5.1.0.GA-src/thirdparty/jboss/messaging/lib/jboss-messaging-sources.jar
You can find most sources for third-party software that makes up the AS in the thirdparty directory within a source JAR file.
thanks Peter, you're right. Got fooled by the thirdparty/...lib path, we do have in our app a similar path which contains only the binary jars.