[CentOS] Where do I get Centos 5 Kernel sources for compiling drivers?

I have a problem with Centos 5 not detecting my realtek 8111b lan chip 
on a Gigabyte GA-945GM-S2 motherboard.
I have found a reference to a new driver from realtek and/or redhat - 
which has instructions on building the driver.
When I go to "make clean modules" I get errors - and it appears to be 
looking in /usr/src/kernels/... for source files which I don't seem to 
have in my centos 5 installation.

Presumably I need to install some kernel source package. If so - where 
do I find it?

Thanks.

Richard.