I usually buy them from my local dealer, but if I'm on the road and I'm near my 2500 change I just pop into whatever the closest dealership is on my drive and pick up a filter. A lot of the middle of nowhere dealerships are great to stop at because they don't get too many BRZ's on the lot and someone always ends up talking to me about it.
I have a Perrin air filter which I clean out during my 5k oil changes.