Where is the cabin air filter location in a Subaru Forester?

Note that not all models have cabin air filtration.

With that said, the manual says that the filter is located behind the glove box. This requires you to open the glove compartment, disconnect the damper (the little piston that allows the door to open slowly and not just fall open) and detach the door stops at the back of the compartment. This exposes the cover for the cabin filter, which can then be slid out and the filter cleaned or replaced.