If you need a custom shape, look at Dar Shelton's web site.
He can make wonderful custom dies and he used to at least run a
service where he keeps the dies, cuts out shapes, and sends them to
you as ordered.
Kevin Potter also makes standard shapes in dies. His require 20g or
thicker metal, so it depends on what you are trying to do.
JB Whitney has a die/punch sets which will do several kinds of sizes
and standard shapes ;you can get the or like sets off Amazon,, eBay
or other googled sites.