How do I install a newer version of Apache2 if apt-get does not automatically find it?

When using package repositories, you're at the mercy of the repository managers for upgrades. In the vast majority of cases, this is a very good thing, as they do a lot of testing on packages and interactions between packages before releasing a new revision into the repo. This prevents you from shooting yourself in the foot in many ways.

If you really need bleeding-edge versions, you'll need to either install from source (not recommended) or find a third-party repository that has more recent versions (though by doing this, you're moving "off-script" and will be more likely to run into bugs or odd interactions between packages).