The official download site for the Linux kernel is kernel.org. You can also get kernel source through your package manager, although keep in mind that Red Hat Linux is an old and outdated distro, so you probably won't be able to get an up to date kernel with it. In fact, I'd recommend upgrading to Fedora or Red Hat Enterprise Linux, or simply moving on to another distro. Especially when you're developing stuff for the kernel, it's important that you develop using modern interfaces and technologies.