If you go into the Mandrake control center, there's a section for setting the locations of your nearest Mandrake rpm mirror. Set these, update your sources list, and gnucash etc. should all appear on your "software available for installation" list.
Alternatively, you can search through
rpmfind.net for those same rpms, download them to a known directory, and install them from either the Control Center, or from the shell (rpm -i <rpm_name>); though the first method is usually preferable. Either way, I certainly wouldn't bother creating a fresh installation CD if you already have a working system, minus those four packages.
Cheers,
mj