You could try room air cleaner replacement filters, cut to size. I got lucky and found some 4" round ones that are great. not the "hepa" filters but the white felt (ish) ones.
An old T-shirt cut to size.
A dust filter face mask from a hardware store.
panyhose.
All the above are less expensive then the ones specificly made for a case fan, and they work better too!
You could use a can of air to blow the dust out, or a vacum to suck it out too!