I remember bluefront was always on these forums talking about air filtration, and how the hardest thing was to find pleated filters that fit a 120mm fan. Well i found a site that will make you 1" thick pleated filters in any size you want. They sell them in a 6pack or 12pack. Each filter lasts 3 months.
Now obviously the price is going to vary a bit depending on the size you specify and the type of filter you choose. But i put in a Pleated MERV 7, with demensions of 4.75" x 4.75" x 1", which is the size of a 120mm fan. A 6pack costs $47.95 and a 12pack costs $82.63. A pretty fair price if you ask me. I figure MERV 7 will give the best airflow out of the choices they offer and still be overkill since all i want to do is filter large dust bunnies and hair.