Hi, Colin (@coling). I have a portable spray booth that I got from Micromark several years ago. When it was time to replace the filters that came with the booth, I got a set of five from Micromark. As you already know, they were pretty expensive. Since then, I have tried two different methods to replace the filters. First, you can get filter material for air conditioners at home improvement stores like Home Depot or Lowes, or the equivalent where you live. You will need to cut them to size. Most of them are more porous than the paint filters, but if you look around you can find something close. The last replacement I got was from Amazon where I found a roll of filter material that will last for a long time. It is similar to the paint filters and the price was very low. You have to cut it to size, but it seems to work well. Good luck.