I know a lot of people have cut or made filters to replace the $37 to $50 (yikes!) factory cabin air filter... but I've found a couple of standard air filters that fit and work great!
Purolator - Part # 33590 - $5.99 from Pep Boys
Fram - Part # 3916 - $6.99 from Pep Boys.
Both fit with no modification - just remember, rubber side down, as the air flows from top to bottom!!
Replacement should be about every 10-15,000 miles.
You have to take out all the screws around the inside of the glove box, two on the bottom and one on the right side of the dash.