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.

http://www.airfilterusa.com/custom_filters.php

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.

That air filter media is excellent, but it can be cut-to-size very easily with common scissors/shears. It is usually reinforced with a soft wire backing mesh, but it really does little damage to decent-quality scissors.

Large panel (14"X24"X1", etc.) filters are so cheap (Walmart. etc.) that I wouldn't order custom sizes. One large panel is at least a years supply.

Hope this helps!