Metal snaps ("poppers", "press studs") are available at most fabric shops, so are a popular choice for use on cloth pads. Because there is no need for an expensive snap press machine (unlike that needed with plastic snaps, they are more cost effective to use.
Depending on the style of snap, they are applied with a hammer, by hand sewing them on or applied with a specialised tool.
Metal snaps do have some disadvantages. The varieties that are not sewn on can fall out if not set correctly, which usually leaves damage to the fabric. They may also rust and be unsuitable for those who have allergies to nickel.
Metal snaps are available in several styles. See also plastic snaps.