Yes you can purchase filters for fans. They are relatively cheap too.
A good tip for keeping out the dust is to have a positive pressure airflow set-up. This means you want more air being sucked in than being blown out. This positive pressure forces air out all the little nooks and crannies in your case effectively sealing it up. A typical positive pressure set-up would consist of two intake fans and only one exhaust. Another good tip is to have your fans spinning as slow as possible, but without significantly effecting your CPU/GPU temps. This limits the amount of air that comes in which in turn will limit the amount of particles ingested by your system. This can be achieved by a fan controller or some motherboards support fan...