For the best accuracy, it is necessary for the external magnet to be properly aligned with the magnetic center of the sensor chip. Also, minimize the distance between the sensor chip and the magnet. Any materials between them must be non-magnetic. Normally, ambient magnetic fields will not affect performance, but it is a good practice to mount the sensor as distant from them as practical. Good quality, low cost rare earth magnets are available from a variety of suppliers. Contact TE Connectivity for recommendations.