Build your own. Go to the hardware store and look for the most common/cheap size furnace filter, and build it to use those. That way you are not stuck having to buy a proprietary filter that, if the company decides to discontinue, you are screwed.
That's what I plan to do when I build my server room, as I want the air in there to be super clean so the servers don't get all dusty. Probably going to put my workstation down there too and run long cables. I have to clean that box like every week because of my cat and she's short haired. Does not help that she often sleeps on top of it though. Silly cat.