What it is in kernel 2.6 you want? If you want 2.6 simply because its the latest and you have no problem with your hardware on RH9, then I suggest to reconsider.
Most recent kernel source can be downloaded from kernel.org. You'll need to compile them yourself. Upgrading your RH9 box to kernel 2.6 will not be trivial at all, since the kernel itself may require many other software to be upgraded first on your RH9 (ex compiler). If you have multiple systems to upgrade, then why not consider upgrading them to a more recent distro?
Another possibility is to hunt for rpms made specifically for rh9. There is a chance (how big of a chance I don't know) someone already went through that trouble.
One thing I am quite sure is you can upgrade to a more recent 2.4 version through the usual up2date / yum / apt-get channels (if you haven't already). This is by far the easiest thing to do, and it is the best solution unless you have very good reasons to move to 2.6.