Sorry just mentioned a few stores of where you might find them, you can get them lots of places similar or find them online.
The filters for heat registers is just made of a good material to catch dust and be cleaned, i'd use them over the scotch bright pads tho(they might be a good idea if you can't find any). The fiber material used in those fan grills above that you can buy work pretty well too and you can just keep cleaning them out with water. Both allow more airflow than a paper towel. However I have known a few people to use a piece of cloth, and one had found a fabric softner type material that was for a filter and worked decently and wasn't all gunked up with fabric softner and allowed better airflow.
I'd see which offers you more airflow and stick with that. BTW most computers have the back fan as the exhaust because it takes away all that hot air from the HSF on your processor. Have you ever revered your fan?