You can find these magnets in the magnet section of a hardware store or a craft store. They are typically sold in packs of 6 or 8. If you cannot find any, get a set of magnetic jewelry clasps from the beading section of a craft store instead. You can use smaller magnets too, but don't use larger ones. Do not use the standard "black" magnets either; they are not strong enough.

Summary:
Get a pair of 14in (6.4mm) silver neodymium magnets.