Here is an answer courtesy of hemicuda on RepairPal:
"the cabin filter is locate on the passenger side in the glove box. to get to it you must remove the shelf in the glove box by simply tugging it from the left side, then you will see at the back of the glove box a section that you can remove. once removed you will see a white compartment with a removable front, remove that and you'll see the cabin filter."
As far as finding the filter, your local auto parts store should carry it. Call your local Autozone/Kragen/O'Reily/NAPA/etc. and tell them the part you're looking for to see if they have it, or if they can order it.