Do you live in a place where forced-air heating is common? If so, you should be able to buy filters wherever people go to buy miscellaneous hardware for houses. In the US, they are rated on a scale called MERV. In other countries, other rating systems are used. You want something at or near the high end of the scale — getting near 100% removal with, for example, a real HEPA filter is unnecessary and potentially counterproductive, but getting to at least, say, 30% reduction in fine particles per pass is important. In the US, this is around MERV 13.