Doing an advanced search of the following site:
http://rpm.pbone.netand looking for kernel-2.6.24 for Mandriva 2008 only shows unofficial results at present.
Another site you can check is
http://www.rpmfind.net to search for them, however, I don't see any here either. Perhaps you'll have have to wait a while.
Alternatively, add the urpmi sources using easyurpmi and make sure you tick the compressed index option - that way the lists are very small and will be easily downloadable on a 64K link. Then, you can simply do this which is much easier:
CODE
urpmf --name kernel | grep 2.6.24
and that will filter the results for what you're looking for. I suggest you add the backports repos too, as these tend to have the latest and greatest which is where the latest kernels are likely to appear.
However, I don't see a need to upgrade to a newer kernel unless you have hardware that is currently unsupported by your existing kernel or you have problems with some hardware not working correctly. If it's working, then leave it as it is.