Just download the source tarball eclipse-cvs.tgz
from here
EDIT: This also includes version history, so it may be larger than you need. For just a current version download Platform-SDK.3.5.2 ~ 100MB
It includes the sources in jar files e.g.
org.eclipse.ui.workbench_3.5.2.M20100113-0800.jar
org.eclipse.ui.workbench.source_3.5.2.M20100113-0800.jar