I wondered the exact same thing when I was replacing mine. I ended up
ordering mine through H-and-A accessories
(http://www.handa-accessories.com/civmaint01.html), although it is not
Genuine Honda brand, it is Micron Air brand. Micron Air did not have a
separate one for left and right side.

When I took out the original cabin filters, I could not see any
difference. #11 and #15 are different because the tabs are mirror
images. But I can't understand how #12 and #13 are different. Maybe you
can call your Honda dealership parts shop and ask.

They are indeed identical, the ONLY difference in my Civic is the
position of the tab on the filter's HOLDER.  Really would not matter
if one was a SPECIAL filter and the other one was plain. All they
really do is to keep stuff from getting into the blower housing and
into the vent ducting (leaves, large particals of dust and dirt). They
do not keep "bad" air from getting into the car (exhaust or smoke),
nor do keep fine dust from getting in (like if you were caught in a
dust storm).

Also if they (the filters) are convered with crud, leaves, twigs and
the like, the very act of moving them, will tend to cause whatever  is
above them, to fall into the ducting, when you pull them out, so best
to make sure you change them at least once a year, and if possible
make sure that you keep the fresh air vents outside the car (near the
windshield), clear of things that can fall into the are where the
filters are, to help reduce the amount of crap that will be on the
filters.