Nickel-plated steel spots are a shiny silver color, with 1/4″ long prongs on the back.  These seem to work best when the total thickness you’re putting them through is about 1/8″ or less, so you’ve got some room to bend the little prongs over.  Extra Small is 4mm in diameter.  Small = 5mm.  Medium = 7mm.  Large = 8mm.  DEC-906  Sold in packs of 100.

Nickel Spots with prongs on Back in Many Sizes for Leather Craft or Fabric Projects