In Slackware, the file you're looking for is "kernel-source-2.4.24-noarch-1" which is located in the "K" series of software. Depending on when you obtained your installation files, the version number may be different. You should be able to install it using pkgtool or installpkg from your original installation media.
EDIT: never mind ... you have Slackware listed in your user profile, but mention in your post that you're running Fedora. Reading comprehension > me. The rest of the post might actually be of use though ...
You say you have "something" in /usr/src ... care to elaborate? The package installs to "/usr/src/linux-2.4.xx". You may try simlinking "linux" to that directory. For example ...
If your linux source is located in /usr/src/linux-2.4.24 ...
ln -s linux-2.4.24 linux
This will make your kernel sources available at /usr/src/linux as well, which is where they're often expected to be.