For onesies try the local hardware dealer (such as ace) that has the pull out draws of hardware. You will pay a premium, but if you are only making a few plugs you do not need ot buy a box of 100.
Like GF said, for just a few plugs try West Marine for a #2 stainlees screw. If they only have #4 you will just need to drill the lip hole a little larger.