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Get a pair of 1⁄4 in (6.4 mm) silver neodymium magnets.
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.