There are a few ways. The easy way is to use apt-get. Type
apt-cache search kernel-image to see what kernels are on offer and then download the one you want. The harder - but recommended - way is to get the kernel source and compile it yourself. I assume when you say "linux 2.6" you're referring to the kernel source.
If you want to compile your own there's a really good walkthrough
here. It's for a 2.4 kernel but the theory is mostly the same. I personally don't like the making of a debian package idea but I have done it before and it does work. With the 2.6 kernels you also have to make sure you get the module-init-tools package.
