Comment 7 for bug 534175

On lucid I can do
  sudo apt-get install dpkg-dev
  apt-get source linux-image-2.6.31-10-rt
On karmic the version is 2.6.31-9. Should be nearly the same kernel, I suppose.

However if you prefer wgetting the lucid tarballs, I believe the correct files to fetch are:
  http://archive.ubuntu.com/ubuntu/pool/universe/l/linux-rt/linux-rt_2.6.31.orig.tar.gz
  http://archive.ubuntu.com/ubuntu/pool/universe/l/linux-rt/linux-rt_2.6.31-10.153.diff.gz

I'm sure there must be a git branch in some repository somewhere containing these sources, but I don't know what that is.