Get the kernel-source from Redhat. you should find it in the same ftp folder where kernel-2.4.18-5 is located.
Don't worry about the architecture for the sources (i386, i586, i686, SMP, athlon) it should work regardless of your setup. Just make sure it is the same version as you kernel (2.4.18-5).
UPDATE: New kernel files at Redhat: 2.4.18-10 Don't forget to install the kernel-source as well.