I don't think getting one from kernel.org is going to work very well as Mandrake puts quite a few patches in their kernel RPMs. So even if you get the exact same version from kernel.org it will be different to your running kernel and might cause errors trying to compile stuff against it.
alltime goto
http://easyurpmi.zarb.org and setup access to the software repositories. Now go menu->System->Configuration->Packaging->Install Software (or go into the Mandrake Control Centre->Software) and install the package 'kernel-source' (without the quotes).