gguna- welcome- stop over by the intro thread and say hi.
To answer your question, you take a bit of steel, chuck it in a lathe, turn it down- and file or mill it to shape, and then heat treat it. Or you call a gun parts dealer and order one.
However, the shape will vary depending on which of over a thousand different 9mms you mean. So- whatcha got- and what happened to the original firing pin?