You can use anonsvn to download a part of the sources. You can first find the location in svn with "websvn". For example, if you want the latest stable version of ksysguard, the place in websvn is
http://websvn.kde.org/branches/KDE/3.4/kdebase/ksysguard/, the latest released version is
http://websvn.kde.org/tags/KDE/3.4.1/kdebase/ksysguard/ .
After that, you can use subversion in combination with the anonsvn server to download the source. The url about the usage of anonsvn is in
http://developer.kde.org/source/anonsvn.html .
Note that you can't compile a subdirectory of a package easily, because it doesn't have bulid system (it doesn't contain a ./configure).