And once you got those source RPMs you can install them with
They should appear in /usr/src/RPM/ and the actual source-code for the packages will be in .tar.bz2 files in the SPECS directory.
Most people find it safer and more convenient to setup and rpm directory tree in their home directory if they are going to work on building or playing with source RPMs.
You should read the Mandriva RPM Howto http://qa.mandriva.com/twiki/bin/view/Main/RpmHowTo
here, or at least the 'Perliminary Tasks' section to explain how to get the directories and rpm config file setup.