I'm sure this is a no-brainer, but I could not find filters small enough to flow into my Paasche airbrush without a funnel, so I bought the standard size automotive filters and cut then in half.

All it takes is a hot melt glue gun, run a large bead up the paper divider, and into the tip and press the filter together until it cools.  Then simply cut it in half.  The smaller filter fiter the mouth of the small airbrush paint container.