What material are plug pins made out of?
For mains power plugs, there is nothing magic about the materials. Many plugs use essentially raw brass (which has a gold-like tone). Or the ones that have a silver-looking tone likely have a nickel plating which prevents the brass from tarnishing and discoloring.

That said, You are quite unlikely to find the kind of pins used in mains plugs as commodity items.  Unless you are manufacturing a product by the 10s of thousands. One major reason is that anything that connects to the mains must be meet strict requirements and have expensive testing before it can be approved for sale.  If you want pins for a mains plug, then just rip the pins out of a mains plug. That is really the only practical option except perhaps making them yourself.