You can buy microfiber filter and cut to size. It is a much better filter than the foam that comes with the projector. That is what most of the guys that own the Viewsonic Pro8100 do. Here is a post with a link. http://www.avsforum.com/avs-vb/showt...9#post18230949 Look under insufficient dust filter work around.
I replaced one in an old Panasonic with a vent filter (that goes at the outlet, not at the furnace) I think it was also electrostatic for dust collection. I could cut one into about 10 small filters, and can toss it out every few months.
for a quick fix go to Home depot Lowe's or maybe even wally world and look for frost king Polyurethane Air conditioner filters. They are 30 PPI (pours per inch), Ideally you want a filter that is 100 PPI. If you have the projector permanently mounted I would look for a square shaped auto air filter available anywhere. They are by FAR the best if you can fit the projector with one. I have a Fram on my Sanyo and its great. It lasts a LONG time cleans easily and allows for great air flow