Looking at the version info of the jars shipped in JBoss AS 5.1.0 http://anonsvn.jboss.org/repos/jbossas/tags/JBoss_5_1_0_GA/component-matrix/pom.xml it's using 2.1.3 version of jboss-web:jasper-jdt:
<dependency>
<groupId>jboss.web</groupId>
<artifactId>jasper-jdt</artifactId>
<version>${version.jboss.web}</version>
</dependency>
which can be found here https://repository.jboss.org/nexus/content/repositories/releases/jboss/web/jasper-jdt/2.1.3.GA/. But I don't know why it's missing the source jar file in that repo.