Why can't I find 2.6 kernel images in the wheezy backports for Debian Wheezy?
Does anybody knows any unofficial mirrors I can just add to apt?
The Debian Backports repository provides you with newer versions, not older.
The default Wheezy kernel is a 3.2 kernel. Hence any kernel packages in wheeyz-backports will be something newer than that. Currently the most recent kernel in wheezy-backports appear to be a 3.14 kernel.
If you really need 2.6 kernels the easiest is probably to reuse 2.6 kernels from Squeeze, which ought to work just fine with in a Wheezy environment. Given the Squeeze-LTS project they might even continue to be supported for a while longer.
Most likely you can simply add the regular Squeeze repos to your sources lists. Given that they contain older versions of everything you shouldn't have to worry about any apt repo pinning.