kernel.org has a public (read-only) git repository that you can clone. It has also tags for every kernel version, so you can checkout a specific version:
# Clone the kernel to your local machine
$ git clone git://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git
$ cd linux-stable
# Find the tag for the version you want
$ git tag -l | grep 3.18.1
v3.18.1
# Create a new branch with that tag
$ git checkout -b my3.18.1 v3.18.1
Now the linux-stable
directory will have the kernel version you wanted.
(I know you said v3.18.1 doesn't exist as a tag, but it does)
Edit: The asker says he was using Torvalds' github repo, which is why he could not find the tag for 3.18.1. This is expected, because Torvalds' github repo only contains tags for release candidates.
The asker is asking for a minor revision (the .1 in 3.18.1), so he needs to get that from https://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git/