Although these magnetic fasteners are referred to as "black" nickel there is not really a difference colourwise between the black and the regular nickel. Please bear that in mind if you are looking or something very dark. 
These magnetic fasteners are mainly used for bags, hand-bags, purses and much more. 
In line with the legs on the magnet, mark cross-sections and cut the cloth.
Then pull the magnet legs so that they pass through the cloth and peek at the back side.
Put the metal counterclock on the legs, press against each other and pull the legs apart. Put the second part in the same way.
Diameter: 18 mm
Total thickness: 2,5 mm
Priced per bag - each bag contains 5 fasteners.