You can order neodymium magnets from www.dealextreme.com (I confess, I am an addicted deal extremist!).
1- A NERF Gun capable of firing stock whistler darts.
2- N whistler dart
3- 3xN 8 mm diameter neodymium magnets. (10mm works too but it is hard to fit through whistler hole)
1- Take one magnet
2- Squeeze the rubber head while pushing the neodymium magnet into whistler hole.
3- Continue steps 1,2 until you are satisfied, in my case 3 was enough, but you can fill up to 5 or more (the more you fill the heavier will be the darts which degrade the range.)
4- Lock, load and fire your dart to a magnet friendly metal plate (like the surface of refrigerator )